[PATCH] drm/ttm: Use ttm_bo_default_io_mem_pfn if io_mem_pfn is NULL

[Christian König]

Am 29.11.2017 um 16:44 schrieb Sean Paul:
> On Wed, Nov 29, 2017 at 04:40:18PM +0100, Christian König wrote:
>> Am 29.11.2017 um 16:36 schrieb Sean Paul:
>>> On Wed, Nov 29, 2017 at 04:24:21PM +0100, Christian König wrote:
>>>> Am 29.11.2017 um 16:20 schrieb Michal Srb:
>>>>> The io_mem_pfn field was added in commit ea642c3216cb2a60d1c0e760ae47ee85c9c16447
>>>>> and is called unconditionally. However, not all drivers were updated to set it.
>>>>>
>>>>> Use the ttm_bo_default_io_mem_pfn function if a driver did not set its own.
>>>>>
>>>>> Signed-off-by: Michal Srb <msrb at suse.com>
>>>> NAK, when we have drivers missing this we should set this in the driver and
>>>> not add the workaround here.
>>> Why? What's the benefit in adding the same hook in 10 drivers (I was hoping
>>> Michal would also remove the token .io_mem_pfn = ttm_bo_default_io_mem_pfn from
>>> those as well)?
>> Not have this extra check in common TTM code? That is seriously bad coding
>> style.
>>
> [citation neeeded]
>
> I'd argue it's bad form to require everyone add a copypasta hook to their driver
> everytime an unrelated driver needs more control than the default gives :-)

Well that is another design problem of TTM that it works like a middle 
layer instead of a tool. But I'm seriously not going to fix this anytime 
soon.

> I think your point would hold if we were talking about a couple of drivers, but
> once you scale to 10+, it's no longer a trivial amount of code. If the null check
> is too distracting, you could always pull it out into an inline.
Yeah, that is one possibility that would work for me as well.

TTM has patterns like the following:

First occasion:
if (a_is_available)
     do_a();
else
     do_b();

Same code a bit lower:
if (a_is_available)
     do_a()
/* Else is missing here */

I'm seriously favoring updating a dozen drivers just to avoid more of this.

If you want to fix that with adding wrapper functions to call the driver 
callback and give a consistent operation when the callback isn't 
available then I'm perfectly fine with that as well. Patches are always 
welcome.

Christian.

>
> Sean
>
>
>> We have tons of fallbacks like this and I unfortunately even created new one
>> without realizing that this is a problem.
>>
>> But this really clutters the TTM code and makes it harder to understand.
>>
>> Christian.
>>
>>> Sean
>>>
>>>> Which driver is missing this? I thought I fixed all of them.
>>>>
>>>> Regards,
>>>> Christian.
>>>>
>>>>> ---
>>>>>     drivers/gpu/drm/ttm/ttm_bo_vm.c | 5 ++++-
>>>>>     1 file changed, 4 insertions(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/drivers/gpu/drm/ttm/ttm_bo_vm.c b/drivers/gpu/drm/ttm/ttm_bo_vm.c
>>>>> index c8ebb757e36b..e25a99bc519d 100644
>>>>> --- a/drivers/gpu/drm/ttm/ttm_bo_vm.c
>>>>> +++ b/drivers/gpu/drm/ttm/ttm_bo_vm.c
>>>>> @@ -234,7 +234,10 @@ static int ttm_bo_vm_fault(struct vm_fault *vmf)
>>>>>     		if (bo->mem.bus.is_iomem) {
>>>>>     			/* Iomem should not be marked encrypted */
>>>>>     			cvma.vm_page_prot = pgprot_decrypted(cvma.vm_page_prot);
>>>>> -			pfn = bdev->driver->io_mem_pfn(bo, page_offset);
>>>>> +			if (bdev->driver->io_mem_pfn)
>>>>> +				pfn = bdev->driver->io_mem_pfn(bo, page_offset);
>>>>> +			else
>>>>> +				pfn = ttm_bo_default_io_mem_pfn(bo, page_offset);
>>>>>     		} else {
>>>>>     			page = ttm->pages[page_offset];
>>>>>     			if (unlikely(!page && i == 0)) {
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