linux-kernel - Re: [PATCH v3 1/3] drm: add SimpleDRM driver

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Date:	Wed, 17 Aug 2016 11:30:38 +0200
From:	Daniel Vetter <daniel@...ll.ch>
To:	Noralf Trønnes <noralf@...nnes.org>
Cc:	dri-devel <dri-devel@...ts.freedesktop.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 1/3] drm: add SimpleDRM driver

On Tue, Aug 16, 2016 at 9:38 PM, Noralf Trønnes <noralf@...nnes.org> wrote:
>> That's still a lot for what amounts to reimplementing mmap on shmem, but
>> badly. What I mean with redirecting is pointing the entire ->mmap
>> operation to the mmap implementation for the underlying mmap. Roughly:
>>
>>         /* normal gem mmap checks first */
>>
>>         /* redirect to shmem mmap */
>>         vma->vm_file = obj->filp;
>>         vma->vm_pgoff = 0;
>>
>>         return obj->filp->f_op->mmap(obj->filp, vma);
>>
>> Much less code ;-)
>
>
> obj->filp is NULL in my case.
>
> And looking at the docs, that's expected since I have driver specific
> backing?
>
> /**
>  * @filp:
>  *
>  * SHMEM file node used as backing storage for swappable buffer objects.
>  * GEM also supports driver private objects with driver-specific backing
>  * storage (contiguous CMA memory, special reserved blocks). In this
>  * case @filp is NULL.
>  */

Hm, I totally misread the driver code. I assumed that we'd just
allocate normal shmem gem objects, and then copy them on-demand onto
the frontbuffer (in the dirty or plane update callbacks). Essentially
treat the firmware fb area as a manual upload display, except that we
don't use i2c or spi to do the upload, but normal mmio writes. I think
that would greatly simplify the driver, and more important: It would
also work like any other kms driver. Currently sdrm is violiting the
spec a bit by aliasing all dumb buffers to the same underlying backing
storage, and that's a bit evil.

The other bit I noticed (and why I was confused): The prime import
code reinvents a lot of wheels, and it digs into the backing storage
directly. Instead it should just call dma_buf_vmap/dma_buf_vunmap and
let the exporter figure out how it works.

Sorry I was all confused here and didn't realize what's going on :(
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch