Re: [PATCH] drm/fb_cma_helper: Implement fb_mmap callback

From: Robin Murphy
Date: Thu Mar 17 2016 - 06:52:56 EST

On 16/03/16 19:14, Russell King - ARM Linux wrote:
On Wed, Mar 16, 2016 at 04:28:25PM +0100, Daniel Vetter wrote:
On Wed, Mar 16, 2016 at 02:57:49PM +0000, Robin Murphy wrote:
In the absence of an fb_mmap callback, the fbdev code falls back to a
naive implementation which relies upon the DMA address being the same
as the physical address, and the buffer being physically contiguous
from there. Whilst this often holds for standard CMA allocations via
the platform's regular DMA ops, if the allocation is provided by an
IOMMU then such assumptions can fall apart spectacularly.

To resolve this, reroute the fb_mmap call to the appropriate DMA API
implementation, as per the other cma_helper calls.

Signed-off-by: Robin Murphy <robin.murphy@xxxxxxx>

Hi dri-devel,

This is an empirical fix for something I tickled via the newly-added
ARM HDLCD driver on a Juno platform - I have no idea whatsoever about
how "proper" it is in terms of the DRM infrastructure, so feel free to
treat this as a bug report rather than an actual patch if appropriate ;)

I think the best case would be if we could have a generic fbdev helper
that remaps to dumb mmap support. But that's a bit tricky to pull of:
1. from fb_info we can get at the fbdev drm_framebuffer.
2. from a drm_framebuffer we can get at the underlying backing storage
object using fb->funcs->get_handle.
3. With that handle we could go into the dumb mmap support (using als the
vma) and create the mmap.

Except that ->get_handle needs a file_priv, and that just exist for the
fbdev emulation kms client. I guess we could fix that by creating a
minimal fake drm file_priv for the fbdev emulation (and treat it more like
any other kms client), but I think that's way too much work when this
simple patch here gets the job done.

I think first, a different question needs to be answered:


struct fb_fix_screeninfo {
char id[16]; /* identification string eg "TT Builtin" */
unsigned long smem_start; /* Start of frame buffer mem */
/* (physical address) */

Should a DMA address be exposed through smem_start, rather than a
physical address as the long-standing documentation quoted above
has stated?

Is it, in fact, a driver bug to store something that isn't a physical
address there?

We could also go into whether it's right to store even a physical address in something which isn't necessarily big enough...

After the time I spent in the bowels of the fbdev code figuring out this crash, I fear that if we dig too deep we may awaken something in the darkness ;)