linux-kernel - Re: [PATCH 3/3] drm/ingenic: Alloc cached GEM buffers with dma_alloc

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Date:   Thu, 1 Oct 2020 07:35:09 +0200
From:   Christoph Hellwig <hch@....de>
To:     Paul Cercueil <paul@...pouillou.net>
Cc:     Christoph Hellwig <hch@....de>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Dave Airlie <airlied@...ux.ie>,
        DRI <dri-devel@...ts.freedesktop.org>,
        Linux Next Mailing List <linux-next@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/3] drm/ingenic: Alloc cached GEM buffers with
 dma_alloc_noncoherent

On Wed, Sep 30, 2020 at 07:16:44PM +0200, Paul Cercueil wrote:
> It turns out that if you want to mmap GEM buffers fully cached, then
> they should be allocated as such as well. Who would have known?
> 
> Introduce a custom .dumb_create callback, that will behave just like
> drm_gem_cma_dumb_create(), except that it will allocate the GEM buffer
> using dma_alloc_noncoherent() if non-coherent memory is what we want.

I think you want to merge this with patch 2, then change patch 2 to
still use dma_alloc_wc for the !ingenic_drm_cached_gem_buf and to
get the phys address using virt_to_phys for the
ingenic_drm_cached_gem_buf instead of abusing the dma address in
->paddr.  The free side also needs to use
dma_free_noncoherent.

> +static int ingenic_drm_gem_cma_dumb_create(struct drm_file *file_priv,
> +					   struct drm_device *drm,
> +					   struct drm_mode_create_dumb *args)
> +{
> +	/*
> +	 * This is basically a copy of drm_gem_cma_dumb_create, which supports
> +	 * creating fully cached GEM buffers.
> +	 */

What about adding this to the core GEM code instead?  We'll probaby
run into other potential users as well.