[PATCH v3 0/2] drm: Add shmem GEM library

Noralf Trønnes noralf at tronnes.org
Fri Sep 14 16:51:25 UTC 2018

Den 14.09.2018 18.13, skrev Daniel Vetter:
> On Fri, Sep 14, 2018 at 5:48 PM, Thomas Hellstrom <thomas at shipmail.org> wrote:
>> Hi, Noralf,
>> On 09/11/2018 02:43 PM, Noralf Trønnes wrote:
>>> This patchset adds a library for shmem backed GEM objects and makes use
>>> of it in tinydrm.
>>> When I made tinydrm I used the CMA helper because it was very easy to
>>> use. July last year I learned that this limits which drivers to PRIME
>>> import from, since CMA requires continuous memory. tinydrm drivers don't
>>> require that. So I set out to change that looking first at shmem, but
>>> that wasn't working since shmem didn't work with fbdev deferred I/O.
>>> Then I did a vmalloc buffer attempt which worked with deferred I/O, but
>>> maybe wouldn't be of so much use as a library for other drivers to use.
>>> As my work to split out stuff from the CMA helper for shared use came to
>>> an end, I had a generic fbdev emulation that uses a shadow buffer for
>>> deferred I/O.
>>> This means that I can now use shmem buffers after all.
>>> I have looked at the other drivers that use drm_gem_get_pages() and
>>> several supports different cache modes so I've done that even though
>>> tinydrm only uses the cached one.
>> Out if interest, how can you use writecombine and uncached with shared
>> memory?
>> as typically the linear kernel map of the affected pages also needs
>> changing?
> I think on x86 at least the core code takes care of that. On arm, I'm
> not sure this just works, since you can't change the mode of the
> linear map. Instead you need specially allocated memory which is _not_
> in the linear map. I guess arm boxes with pcie slots aren't that
> common yet :-)

I was hoping to get some feedback on these cache mode flags.

These drivers use them:

I don't know if they make sense or not, so any help is appreciated.


> -Daniel
>> Thanks,
>> Thomas
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