[PATCH 1/2] drm/shmem: Use cached mappings by default

[Thomas Zimmermann]

Hi Gerd

Am 18.05.20 um 10:23 schrieb Gerd Hoffmann:
>>> $ git grep drm_gem_shmem_mmap
>>>
>>> We also need correct access from userspace, otherwise the gpu is going to
>>> be sad.
>>
>> I've been thinking about this, and I think it means that we can never
>> have cached mappings anywhere. Even if shmem supports it internally for
>> most drivers, as soon as the page are exported, the importer could
>> expect uncached memory.
> 
> The importer should not expect anything but call dma-buf ops so the
> exporter has a chance to handle this correctly.

I have the following case in mind: Suppose the exporter maps cached
pages and the importer expects uncached pages for DMA. There is
map_dma_buf/unmap_dma_buf, which can implement a cache flush for the
cached pages. Is it guaranteed that the importer calls this around each
DMA operation? If it doesn't, it could miss later buffer updates from
the exporter. And there's apparently no explicit flush op in dma_buf_ops.

Best regards
Thomas

> 
> (Yes, we don't get this right everywhere, some drivers take the dma-bufs
> list of pages and do their own thing ...).
> 
> take care,
>   Gerd
> 
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-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Felix Imendörffer

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