[PATCH v2] drm/print: Introduce drm_line_printer

Thu May 30 09:33:52 UTC 2024

On 30.05.2024 09:49, Jani Nikula wrote:
> On Wed, 29 May 2024, John Harrison <john.c.harrison at intel.com> wrote:
>> On 5/28/2024 06:06, Michal Wajdeczko wrote:
>>> This drm printer wrapper can be used to increase the robustness of
>>> the captured output generated by any other drm_printer to make sure
>>> we didn't lost any intermediate lines of the output by adding line
>>> numbers to each output line. Helpful for capturing some crash data.
>>>
>>> Signed-off-by: Michal Wajdeczko <michal.wajdeczko at intel.com>
>>> Cc: Jani Nikula <jani.nikula at intel.com>
>>> Cc: John Harrison <John.C.Harrison at Intel.com>
>>> ---
>>> v2: don't abuse prefix, use union instead (Jani)
>>>      don't use 'dp' as name, prefer 'p' (Jani)
>>>      add support for unique series identifier (John)
>>> ---
>>>   drivers/gpu/drm/drm_print.c | 14 ++++++++
>>>   include/drm/drm_print.h     | 68 ++++++++++++++++++++++++++++++++++++-
>>>   2 files changed, 81 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/gpu/drm/drm_print.c b/drivers/gpu/drm/drm_print.c
>>> index cf2efb44722c..be9cbebff5b3 100644
>>> --- a/drivers/gpu/drm/drm_print.c
>>> +++ b/drivers/gpu/drm/drm_print.c
>>> @@ -214,6 +214,20 @@ void __drm_printfn_err(struct drm_printer *p, struct va_format *vaf)
>>>   }
>>>   EXPORT_SYMBOL(__drm_printfn_err);
>>>   
>>> +void __drm_printfn_line(struct drm_printer *p, struct va_format *vaf)
>>> +{
>>> +	unsigned int counter = ++p->line.counter;
>> Wrong units, but see below anyway...

it really doesn't matter as it is temporary var used in printf()
actual 'short' counter will wrap on its own unit boundary

>>
>>> +	const char *prefix = p->prefix ?: "";
>>> +	const char *pad = p->prefix ? " " : "";
>>> +
>>> +	if (p->line.series)
>>> +		drm_printf(p->arg, "%s%s%u.%u: %pV",
>>> +			   prefix, pad, p->line.series, counter, vaf);
>>> +	else
>>> +		drm_printf(p->arg, "%s%s%u: %pV", prefix, pad, counter, vaf);
>>> +}
>>> +EXPORT_SYMBOL(__drm_printfn_line);
>>> +
>>>   /**
>>>    * drm_puts - print a const string to a &drm_printer stream
>>>    * @p: the &drm printer
>>> diff --git a/include/drm/drm_print.h b/include/drm/drm_print.h
>>> index 089950ad8681..f4d9b98d7909 100644
>>> --- a/include/drm/drm_print.h
>>> +++ b/include/drm/drm_print.h
>>> @@ -176,7 +176,13 @@ struct drm_printer {
>>>   	void (*puts)(struct drm_printer *p, const char *str);
>>>   	void *arg;
>>>   	const char *prefix;
>>> -	enum drm_debug_category category;
>>> +	union {
>>> +		enum drm_debug_category category;
>>> +		struct {
>>> +			unsigned short series;
>>> +			unsigned short counter;
>> Any particular reason for using 'short' rather than 'int'? Short is only 

didn't want to increase the size of the struct drm_printer and with
little luck sizeof two shorts will be less/equal sizeof enum

>> 16bits right? That might seem huge but a GuC log buffer with 16MB debug 
>> log (and minimum sizes for the other sections) when dumped via the 
>> original debugfs hexdump mechanism is 1,245,444 lines. That line count 

if your capture relies on collecting all 1,245,444 lines then likely you
have other problem that needs solving than line counter overflow

>> goes down a lot when you start using longer lines for the dump, but it 
>> is still in the tens of thousands of lines.  So limiting to 16 bits is 
>> an overflow just waiting to happen.

but even in case of an overflow it should be relatively easy to teach
any parser to deal with line .0 as indicator to restart any tracker

and it is highly unlikely that any captured logs will miss exactly
65,536 contiguous lines, but even then it should be noticeable gap

>>
>>> +		} line;
>>> +	};
>>>   };
>>>   
>>>   void __drm_printfn_coredump(struct drm_printer *p, struct va_format *vaf);
>>> @@ -186,6 +192,7 @@ void __drm_puts_seq_file(struct drm_printer *p, const char *str);
>>>   void __drm_printfn_info(struct drm_printer *p, struct va_format *vaf);
>>>   void __drm_printfn_dbg(struct drm_printer *p, struct va_format *vaf);
>>>   void __drm_printfn_err(struct drm_printer *p, struct va_format *vaf);
>>> +void __drm_printfn_line(struct drm_printer *p, struct va_format *vaf);
>>>   
>>>   __printf(2, 3)
>>>   void drm_printf(struct drm_printer *p, const char *f, ...);
>>> @@ -357,6 +364,65 @@ static inline struct drm_printer drm_err_printer(struct drm_device *drm,
>>>   	return p;
>>>   }
>>>   
>>> +/**
>>> + * drm_line_printer - construct a &drm_printer that prefixes outputs with line numbers
>>> + * @p: the &struct drm_printer which actually generates the output
>>> + * @prefix: optional output prefix, or NULL for no prefix
>>> + * @series: optional unique series identifier, or 0 to omit identifier in the output
>>> + *
>>> + * This printer can be used to increase the robustness of the captured output
>>> + * to make sure we didn't lost any intermediate lines of the output. Helpful
>>> + * while capturing some crash data.
>>> + *
>>> + * Example 1::
>>> + *
>>> + *	void crash_dump(struct drm_device *drm)
>>> + *	{
>>> + *		static unsigned short id;
>>> + *		struct drm_printer p = drm_err_printer(drm, "crash");
>>> + *		struct drm_printer lp = drm_line_printer(&p, "dump", ++id);
>> Is there any benefit or other reason to split the prefix across two 
>> separate printers? It seems like a source of confusion. As in, the code

it's not any kind of the required 'split', as both printers used here
can treat prefix as optional if NULL, but rather a way to show how to
pass two potentially separated prefixes, as one of them could be already
prepared (like engine name or any other alias) or if the primary printer
does not accept any prefix at all (and this a limitation of our existing
custom GT oriented printers [1] [2])

[1]
https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/xe/xe_gt_printk.h#L66
[2]
https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/xe/xe_gt_printk.h#L81

>> will allow a double prefix, there is not much you can do about that 
>> because losing the prefix from drm_line_printer would mean no prefix at 

but why would we loose the prefix from the primary printer ?

>> all when not using drm_err_printer underneath. But why explicitly split 
>> the message across both printers in the usage example? This is saying 
>> that this is the recommended way to use the interface, but with no 
>> explanation of why the split is required or how the split should be done.

the drm_line_printer is flexible and can be used in many configurations,
above is just one of the potential uses that shows full output

> 
> You could have a printer, and then add two separate line counted blocks.
> 
> 	struct drm_printer p = drm_err_printer(drm, "parent");
> 	struct drm_printer lp1 = drm_line_printer(&p, "child 1", 0);
> 
> 	...
> 
> 	struct drm_printer lp2 = drm_line_printer(&p, "child 2", 0);
> 
> 	...
> 
> p could be defined elsewhere and passed into separate functions which
> each have the line printing. The two prefixes can be useful.

didn't considered that, but as stated above, drm_line_printer is
flexible and can be used in many different ways, like this new one

> 
>> Also, there is really no specific connection to crashes. The purpose of 
>> this is for allowing the dumping of multi-line blocks of data. One use 
>> is for debugging crashes. But there are many debug operations that 
>> require the same dump and do not involve a crash. And this support is 
>> certainly not intended to be used on general end-user GPU hangs. For 
>> those, everything should be part of the devcoredump core file that is 
>> produced and accessible via sysfs. We absolutely should not be dumping 
>> huge data blocks in customer release drivers except in very extreme 
>> circumstances.

if you are trying to convince me that we don't need any custom
drm_printer that would take care of tracking and printing line numbers
in kind of more robust way and instead we should be doing such line
prints in a error prone way on it's own as you did in [3], then sorry,
I'm not convinced, unless you just feel that it shouldn't be part of the
drm yet, but then decision is drm maintainer hands (and also in the Xe
maintainers who don't want to fall into i915-ish private solutions trap)

[3] https://patchwork.freedesktop.org/patch/594021/?series=133349&rev=2

> 
> A devcoredump implementation could use a drm_printer too?
> 
> Is this only about bikeshedding the example? I'm not sure I get the
> point here.
> 
>>
>>> + *
>>> + *		drm_printf(&lp, "foo");
>>> + *		drm_printf(&lp, "bar");
>>> + *	}
>>> + *
>>> + * Above code will print into the dmesg something like::
>>> + *
>>> + *	[ ] 0000:00:00.0: [drm] *ERROR* crash dump 1.1: foo
>>> + *	[ ] 0000:00:00.0: [drm] *ERROR* crash dump 1.2: bar
>>> + *
>>> + * Example 2::
>>> + *
>>> + *	void line_dump(struct device *dev)
>>> + *	{
>>> + *		struct drm_printer p = drm_info_printer(dev);
>>> + *		struct drm_printer lp = drm_line_printer(&p, NULL, 0);
>>> + *
>>> + *		drm_printf(&lp, "foo");
>>> + *		drm_printf(&lp, "bar");
>>> + *	}
>>> + *
>>> + * Above code will print::
>>> + *
>>> + *	[ ] 0000:00:00.0: [drm] 1: foo
>>> + *	[ ] 0000:00:00.0: [drm] 2: bar
>> Not really seeing the point of having two examples listed. The first 
>> includes all the optional extras, the second is just repeating with no 
>> new information.
> 
> You see how the "series" param behaves?

exactly

> 
> BR,
> Jani.
> 
>>
>> John.
>>
>>> + *
>>> + * RETURNS:
>>> + * The &drm_printer object
>>> + */
>>> +static inline struct drm_printer drm_line_printer(struct drm_printer *p,
>>> +						  const char *prefix,
>>> +						  unsigned short series)
>>> +{
>>> +	struct drm_printer lp = {
>>> +		.printfn = __drm_printfn_line,
>>> +		.arg = p,
>>> +		.prefix = prefix,
>>> +		.line = { .series = series, },
>>> +	};
>>> +	return lp;
>>> +}
>>> +
>>>   /*
>>>    * struct device based logging
>>>    *
>>
> 