[PATCH v3 0/4] dma-buf: Flag vmap'ed memory as system or I/O memory

[Christian König]

Am 28.09.20 um 09:37 schrieb Thomas Zimmermann:
> Hi
>
> Am 28.09.20 um 08:50 schrieb Christian König:
>> Am 27.09.20 um 21:16 schrieb Sam Ravnborg:
>>> Hi Thomas.
>>>
>>>>> struct simap {
>>>>>          union {
>>>>>                  void __iomem *vaddr_iomem;
>>>>>                  void *vaddr;
>>>>>          };
>>>>>          bool is_iomem;
>>>>> };
>>>>>
>>>>> Where simap is a shorthand for system_iomem_map
>>>>> And it could al be stuffed into a include/linux/simap.h file.
>>>>>
>>>>> Not totally sold on the simap name - but wanted to come up with
>>>>> something.
>>>> Yes. Others, myself included, have suggested to use a name that does not
>>>> imply a connection to the dma-buf framework, but no one has come up with
>>>>    a good name.
>>>>
>>>> I strongly dislike simap, as it's entirely non-obvious what it does.
>>>> dma-buf-map is not actually wrong. The structures represents the mapping
>>>> of a dma-able buffer in most cases.
>>>>
>>>>> With this approach users do not have to pull in dma-buf to use it and
>>>>> users will not confuse that this is only for dma-buf usage.
>>>> There's no need to enable dma-buf. It's all in the header file without
>>>> dependencies on dma-buf. It's really just the name.
>>>>
>>>> But there's something else to take into account. The whole issue here is
>>>> that the buffer is disconnected from its originating driver, so we don't
>>>> know which kind of memory ops we have to use. Thinking about it, I
>>>> realized that no one else seemed to have this problem until now.
>>>> Otherwise there would be a solution already. So maybe the dma-buf
>>>> framework *is* the native use case for this data structure.
>>> We have at least:
>>> linux/fb.h:
>>>      union {
>>>          char __iomem *screen_base;      /* Virtual address */
>>>          char *screen_buffer;
>>>      };
>>>
>>> Which solve more or less the same problem.
> I thought this was for convenience. The important is_iomem bit is missing.
>
>> I also already noted that in TTM we have exactly the same problem and a
>> whole bunch of helpers to allow operations on those pointers.
> How do you call this within TTM?

ttm_bus_placement, but I really don't like that name.

>
> The data structure represents a pointer to either system or I/O memory,
> but not necessatrily device memory. It contains raw data. That would
> give something like
>
>    struct databuf_map
>    struct databuf_ptr
>    struct dbuf_map
>    struct dbuf_ptr
>
> My favorite would be dbuf_ptr. It's short and the API names would make
> sense: dbuf_ptr_clear() for clearing, dbuf_ptr_set_vaddr() to set an
> address, dbuf_ptr_incr() to increment, etc. Also, the _ptr indicates
> that it's a single address; not an offset with length.

Puh, no idea. All of that doesn't sound like it 100% hits the underlying 
meaning of the structure.

Christian.

>
> Best regards
> Thomas
>
>> Christian.
>>
>>>   
>>>> Anyway, if a better name than dma-buf-map comes in, I'm willing to
>>>> rename the thing. Otherwise I intend to merge the patchset by the end of
>>>> the week.
>>> Well, the main thing is that I think this shoud be moved away from
>>> dma-buf. But if indeed dma-buf is the only relevant user in drm then
>>> I am totally fine with the current naming.
>>>
>>> One alternative named that popped up in my head: struct sys_io_map {}
>>> But again, if this is kept in dma-buf then I am fine with the current
>>> naming.
>>>
>>> In other words, if you continue to think this is mostly a dma-buf
>>> thing all three patches are:
>>> Acked-by: Sam Ravnborg <sam at ravnborg.org>
>>>
>>>      Sam
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