linux-kernel - Re: [PATCH] x86/PCI: sta2x11: use default DMA address translation ops

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Date:   Thu, 17 Oct 2019 17:41:20 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
Cc:     rubini@...dd.com, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        hch@...radead.com, linux-kernel@...r.kernel.org,
        linux-pci@...r.kernel.org
Subject: Re: [PATCH] x86/PCI: sta2x11: use default DMA address translation ops

Hi Nicolas,

I'm hoping Christoph will chime in and one of the x86 guys will merge
this, since I'm not a DMA expert.  Trivial comments/questions below.

On Wed, Oct 16, 2019 at 06:51:37PM +0200, Nicolas Saenz Julienne wrote:
> The devices found behind this PCIe chip have unusual DMA mapping
> constraints as there is an AMBA interconnect placed in between them and
> the different PCI endpoints. The offset between physical memory
> addresses and AMBA's view is provided by reading a PCI config register,
> which is saved and used whenever DMA mapping is needed.
> 
> It turns out that this DMA setup can be represented by properly setting
> 'dma_pfn_offset', 'dma_bus_mask' and 'dma_mask' during the PCI device
> enable fixup. And ultimately allows us to get rid of this device's
> custom DMA functions.
> 
> Aside from the code deletion and DMA setup, sta2x11_pdev_to_mapping() is
> moved to avoid warnings whenever CONFIG_PM is not enabled.
> 
> Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@...e.de>

> -	pci_read_config_dword(pdev, AHB_BASE(0), &map->amba_base);
> +
> +	pci_read_config_dword(pdev, AHB_BASE(0), &amba_base);
> +	dev->dma_pfn_offset = PFN_DOWN(-amba_base);
> +	dev->bus_dma_mask = amba_base + STA2X11_AMBA_SIZE - 1;

I think of a mask as typically being one less than a power of two, but
that's not the case here, e.g., STA2X11_AMBA_SIZE - 1 == 0x1fffffff
(512MB-1), so if amba_size is 1G, the mask will be 0x5fffffff.

Just double-checking to be sure that's what you intend.

> +	pci_set_consistent_dma_mask(pdev, amba_base + STA2X11_AMBA_SIZE - 1);
> +	pci_set_dma_mask(pdev, amba_base + STA2X11_AMBA_SIZE - 1);

Maybe add a local variable instead of repeating the "amba_base + ..."
expression three times?

>  	/* Configure AHB mapping */
>  	pci_write_config_dword(pdev, AHB_PEXLBASE(0), 0);
> @@ -252,14 +156,25 @@ static void sta2x11_map_ep(struct pci_dev *pdev)
>  		pci_write_config_dword(pdev, AHB_CRW(i), 0);
>  
>  	dev_info(&pdev->dev,
> -		 "sta2x11: Map EP %i: AMBA address %#8x-%#8x\n",
> -		 sta2x11_pdev_to_ep(pdev),  map->amba_base,
> -		 map->amba_base + STA2X11_AMBA_SIZE - 1);
> +		 "sta2x11: Map EP %i: AMBA address %#8x-%#8llx\n",
> +		 sta2x11_pdev_to_ep(pdev), amba_base, dev->bus_dma_mask);

This would read better as

  amba_base, amba_base + STA2X11_AMBA_SIZE - 1

I know that's the same dev->bus_dma_mask, but a "mask" is not the
obvious name for the end of a range.