lkml.org 
[lkml]   [2013]   [May]   [7]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
Date
SubjectRe: [PATCH 3/7] drm: Update drm_addmap and drm_mmap to use PAT WC instead of MTRRs
On Tue, May 7, 2013 at 7:08 AM, Alex Deucher <alexdeucher@gmail.com> wrote:
> On Mon, May 6, 2013 at 7:39 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>> On Mon, May 6, 2013 at 4:04 PM, Jerome Glisse <j.glisse@gmail.com> wrote:
>>> On Mon, May 6, 2013 at 5:22 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>>>> On Fri, May 3, 2013 at 4:00 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>>>>> Signed-off-by: Andy Lutomirski <luto@amacapital.net>
>>>>> ---
>>>>>
>>>>> This needs careful review. I don't really know what this code does, nor
>>>>> do I have the hardware. (I don't understand AGP and the associated
>>>>> caching implications.)
>>>>
>>>> This patch is wrong (I didn't update the matching mtrr_del), and I'm
>>>> reworking this whole series. But I may need some help on this one:
>>>> why is the mtrr handle of a map (whatever a map is) exported to
>>>> userspace via the ADD_MAP and GET_MAP ioctls? What (if anything) is
>>>> userspace supposed to do with it? Do I need to return a valid MTRR
>>>> register number? Is there any userspace code at all that sets
>>>> _DRM_WRITE_COMBINING in DRM_IOCTL_ADD_MAP with appropriate alignment
>>>> and needs the MTRR, for which the drm driver doesn't already add the
>>>> MTRR?
>>>>
>>>> --Andy
>>>
>>> From memory, even on pat system we need mtrr for VRAM is PCI BAR. We
>>> cover it with a write combine MTRR. The whole ioctl is use by some ddx
>>> or maybe even directly the XServer to do this mtrr mess in userspace.
>>
>> Egads! So we have a _DRM_WRITE_COMBINING flag, which will continue to
>> work fine, but almost nothing uses it.
>>
>> I'm amazed this stuff works in the current code, though. Apparently
>> the memory type (WC or UC) of a drm mapping is determined by the mtrr
>> the driver set, but if one part of the BAR is textures or the
>> framebuffer and another part is an outgoing command ring, won't one of
>> them end up with the wrong memory type?
>
> A lot of old chips used to put the registers and framebuffer in the
> same BAR. IIRC, the xserver and later libpciaccess had workarounds to
> deal with this.

I think I read the code wrong (so my patch is garbage). Maybe there's
actually no problem -- if DRM_AGP and DRM_FRAME_BUFFER are always WC,
DRM_REGISTERS is only WC if explicitly requested, and DRM_SHM is
always WB, so everything should be covered.

Anything using libpciaccess ought to be unaffected by my changes -- I
don't want to change /proc/mtrr or the sysfs stuff. The only possible
issue is if there's a memtype conflict, but that's nothing new.

--Andy


\
 
 \ /
  Last update: 2013-05-07 19:01    [W:0.061 / U:0.312 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site