linux-kernel - Re: [PATCH 1/5] [media] rockchip/rga: v4l2 m2m support

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Date:   Tue, 27 Jun 2017 17:39:09 +0900
From:   Tomasz Figa <tfiga@...omium.org>
To:     Jacob Chen <jacob-chen@...wrt.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "open list:ARM/Rockchip SoC..." <linux-rockchip@...ts.infradead.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Heiko Stübner <heiko@...ech.de>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Hans Verkuil <hans.verkuil@...co.com>
Subject: Re: [PATCH 1/5] [media] rockchip/rga: v4l2 m2m support

Hi Jacob,

Please see my comments inline.

On Mon, Jun 26, 2017 at 11:51 PM, Jacob Chen <jacob-chen@...wrt.com> wrote:
> Rockchip RGA is a separate 2D raster graphic acceleration unit. It
> accelerates 2D graphics operations, such as point/line drawing, image
> scaling, rotation, BitBLT, alpha blending and image blur/sharpness.
[snip]
> +static int rga_buf_init(struct vb2_buffer *vb)
> +{
> +       struct rga_ctx *ctx = vb2_get_drv_priv(vb->vb2_queue);
> +       struct rockchip_rga *rga = ctx->rga;
> +       struct sg_table *sgt;
> +       struct scatterlist *sgl;
> +       unsigned int *pages;
> +       struct rga_buf *buf;
> +       unsigned int address, len, i, p;
> +       unsigned int mapped_size = 0;
> +
> +       /* Create local MMU table for RGA */
> +       sgt = vb2_plane_cookie(vb, 0);
> +
> +       /*
> +        * Alloc (2^3 * 4K) = 32K byte for storing pages, those space could
> +        * cover 32K * 4K = 128M ram address.

Unless I'm missing something, there is 1024 32-bit values in one 4K
page, which can point to 4 MB of memory. The code allocates 8 of them,
which in total allows at most 32 MB per buffer.

> +        */
> +       pages = (unsigned int *)__get_free_pages(GFP_KERNEL | __GFP_ZERO, 3);

This is rather unfortunate and you should expect failures here on
actively used systems with uptime longer than few hours. Changing this
to dma_alloc_coherent() and enabling CMA _might_ give you a bit better
success rate, but...

Normally, this kind of (scatter-gather capable) hardware would allow
some kind of linking of separate pages, e.g. last entry in the page
would point to the next page, or something like that. Doesn't this RGA
block have something similar?

Best regards,
Tomasz