linux-kernel - Re: [PATCH 2/4] drm/prime: document that use the page array is deprecated

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Message-ID: <6cc96549-2724-6849-0f38-0de60ed6989c@gmail.com>
Date:   Fri, 9 Oct 2020 09:36:41 +0200
From:   Christian König <ckoenig.leichtzumerken@...il.com>
To:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linaro-mm-sig@...ts.linaro.org, dri-devel@...ts.freedesktop.org,
        linux-media@...r.kernel.org, chris@...is-wilson.co.uk,
        airlied@...hat.com, akpm@...ux-foundation.org,
        sumit.semwal@...aro.org
Subject: Re: [PATCH 2/4] drm/prime: document that use the page array is
 deprecated

Am 08.10.20 um 16:14 schrieb Daniel Vetter:
> On Thu, Oct 08, 2020 at 04:09:14PM +0200, Daniel Vetter wrote:
>> On Thu, Oct 08, 2020 at 01:23:40PM +0200, Christian König wrote:
>>> We have reoccurring requests on this so better document that
>>> this approach doesn't work and dma_buf_mmap() needs to be used instead.
>>>
>>> Signed-off-by: Christian König <christian.koenig@....com>
>>> ---
>>>   drivers/gpu/drm/drm_prime.c | 7 ++++++-
>>>   1 file changed, 6 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/gpu/drm/drm_prime.c b/drivers/gpu/drm/drm_prime.c
>>> index 4910c446db83..16fa2bfc271e 100644
>>> --- a/drivers/gpu/drm/drm_prime.c
>>> +++ b/drivers/gpu/drm/drm_prime.c
>>> @@ -956,7 +956,7 @@ EXPORT_SYMBOL(drm_gem_prime_import);
>>>   /**
>>>    * drm_prime_sg_to_page_addr_arrays - convert an sg table into a page array
>>>    * @sgt: scatter-gather table to convert
>>> - * @pages: optional array of page pointers to store the page array in
>>> + * @pages: deprecated array of page pointers to store the page array in
>>>    * @addrs: optional array to store the dma bus address of each page
>>>    * @max_entries: size of both the passed-in arrays
>>>    *
>>> @@ -965,6 +965,11 @@ EXPORT_SYMBOL(drm_gem_prime_import);
>>>    *
>>>    * Drivers can use this in their &drm_driver.gem_prime_import_sg_table
>>>    * implementation.
>>> + *
>>> + * Specifying the pages array is deprecated and strongly discouraged for new
>>> + * drivers. The pages array is only useful for page faults and those can
>>> + * corrupt fields in the struct page if they are not handled by the exporting
>>> + * driver.
>>>    */
>> I'd make this a _lot_ stronger: Aside from amdgpu and radeon all drivers
>> using this only need it for the pages array. Imo just open-code the sg
>> table walking loop in amdgpu/radeon (it's really not much code), and then
>> drop the dma_addr_t parameter from this function here (it's set to NULL by
>> everyone else).
>>
>> And then deprecate this entire function here with a big warning that a)
>> dma_buf_map_attachment is allowed to leave the struct page pointers NULL
>> and b) this breaks mmap, users must call dma_buf_mmap instead.
>>
>> Also maybe make it an uppercase DEPRECATED or something like that :-)
> OK I just realized I missed nouveau. That would be 3 places where we need
> to stuff the dma_addr_t list into something ttm can take. Still feels
> better than this half-deprecated function kludge ...

Mhm, I don't see a reason why nouveau would need the struct page either.

How about we split that up into two function?

One for converting the sg_table into a linear dma_addr array.

And one for converting the sg_table into a linear struct page array with 
a __deprecated attribute on it?

Christian.

> -Daniel
>
>> -Daniel
>>
>>>   int drm_prime_sg_to_page_addr_arrays(struct sg_table *sgt, struct page **pages,
>>>   				     dma_addr_t *addrs, int max_entries)
>>> -- 
>>> 2.17.1
>>>
>> -- 
>> Daniel Vetter
>> Software Engineer, Intel Corporation
>> http://blog.ffwll.ch