linux-kernel - Re: [PATCH 3/3] drm/etnaviv: use a 32 bit mask as coherent DMA mask

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Message-ID: <20210826121917.GS22278@shell.armlinux.org.uk>
Date:   Thu, 26 Aug 2021 13:19:17 +0100
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Michael Walle <michael@...le.cc>
Cc:     etnaviv@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org,
        "Lukas F . Hartmann" <lukas@...re.com>,
        Marek Vasut <marek.vasut@...il.com>,
        Lucas Stach <l.stach@...gutronix.de>,
        Christian Gmeiner <christian.gmeiner@...il.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>
Subject: Re: [PATCH 3/3] drm/etnaviv: use a 32 bit mask as coherent DMA mask

On Thu, Aug 26, 2021 at 02:10:06PM +0200, Michael Walle wrote:
> -	pdev->dev.coherent_dma_mask = DMA_BIT_MASK(40);
> -	pdev->dev.dma_mask = &pdev->dev.coherent_dma_mask;
> +	/*
> +	 * PTA and MTLB can have 40 bit base addresses, but
> +	 * unfortunately, an entry in the MTLB can only point to a
> +	 * 32 bit base address of a STLB. Moreover, to initialize the
> +	 * MMU we need a command buffer with a 32 bit address because
> +	 * without an MMU there is only an indentity mapping between
> +	 * the internal 32 bit addresses and the bus addresses.
> +	 *
> +	 * To make things easy, we set the dma_coherent_mask to 32
> +	 * bit to make sure we are allocating the command buffers and
> +	 * TLBs in the lower 4 GiB address space.
> +	 */
> +	if (dma_set_mask(&pdev->dev, DMA_BIT_MASK(40)) ||
> +	    dma_set_coherent_mask(&pdev->dev, DMA_BIT_MASK(32))) {
> +		dev_dbg(&pdev->dev, "No suitable DMA available\n");
> +		return -ENODEV;
> +	}

This makes no sense. In the previous patch, you initialised
pdev->dev.dma_mask ot point at the coherent mask, implying that
it wasn't already set - for which dma_coerce_mask_and_coherent()
should be used. Now you're just calling dma_set_mask(), which will
fail if pdev->dev.dma_mask hasn't already been set to point at
something.

If it's already been initialised to point at something, then you
shouldn't be overwriting it in the driver, and you should've used
dma_set_mask_and_coherent() in your previous patch.

Confused.

-- 
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