linux-kernel - Re: [PATCH 1/1] PCI: layerscape-ep: set 64-bit DMA mask

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Message-ID: <ZQyM+9uxFoX5TgPa@lizhi-Precision-Tower-5810>
Date:   Thu, 21 Sep 2023 14:35:39 -0400
From:   Frank Li <Frank.li@....com>
To:     Christophe JAILLET <christophe.jaillet@...adoo.fr>
Cc:     Minghuan Lian <minghuan.Lian@....com>,
        Mingkai Hu <mingkai.hu@....com>, Roy Zang <roy.zang@....com>,
        Lorenzo Pieralisi <lpieralisi@...nel.org>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Rob Herring <robh@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        "open list:PCI DRIVER FOR FREESCALE LAYERSCAPE" 
        <linuxppc-dev@...ts.ozlabs.org>,
        "open list:PCI DRIVER FOR FREESCALE LAYERSCAPE" 
        <linux-pci@...r.kernel.org>,
        "moderated list:PCI DRIVER FOR FREESCALE LAYERSCAPE" 
        <linux-arm-kernel@...ts.infradead.org>,
        open list <linux-kernel@...r.kernel.org>, imx@...ts.linux.dev
Subject: Re: [PATCH 1/1] PCI: layerscape-ep: set 64-bit DMA mask

On Thu, Sep 21, 2023 at 07:59:51PM +0200, Christophe JAILLET wrote:
> Le 21/09/2023 à 17:37, Frank Li a écrit :
> > From: Guanhua Gao <guanhua.gao@....com>
> > 
> > Set DMA mask and coherent DMA mask to enable 64-bit addressing.
> > 
> > Signed-off-by: Guanhua Gao <guanhua.gao@....com>
> > Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@....com>
> > Signed-off-by: Frank Li <Frank.Li@....com>
> > ---
> >   drivers/pci/controller/dwc/pci-layerscape-ep.c | 5 +++++
> >   1 file changed, 5 insertions(+)
> > 
> > diff --git a/drivers/pci/controller/dwc/pci-layerscape-ep.c b/drivers/pci/controller/dwc/pci-layerscape-ep.c
> > index de4c1758a6c33..6fd0dea38a32c 100644
> > --- a/drivers/pci/controller/dwc/pci-layerscape-ep.c
> > +++ b/drivers/pci/controller/dwc/pci-layerscape-ep.c
> > @@ -249,6 +249,11 @@ static int __init ls_pcie_ep_probe(struct platform_device *pdev)
> >   	pcie->big_endian = of_property_read_bool(dev->of_node, "big-endian");
> > +	/* set 64-bit DMA mask and coherent DMA mask */
> > +	if (dma_set_mask_and_coherent(dev, DMA_BIT_MASK(64)))
> > +		if (dma_set_mask_and_coherent(dev, DMA_BIT_MASK(32)))
> 
> As stated in [1], dma_set_mask() with a 64-bit mask will never
> fail if dev->dma_mask is non-NULL.
> 
> So, if it fails, the 32 bits case will also fail for the same reason.
> There is no need for the 2nd test.
> 
> 
> See [1] for Christoph Hellwig comment about it.

I don't think it is true. the below is dma_set_mask()'s implementation

int dma_set_mask(struct device *dev, u64 mask)
{
	/*
	 * Truncate the mask to the actually supported dma_addr_t width to
	 * avoid generating unsupportable addresses.
	 */
	mask = (dma_addr_t)mask;

	if (!dev->dma_mask || !dma_supported(dev, mask))
				^^^^^^^
		return -EIO;

	arch_dma_set_mask(dev, mask);
	*dev->dma_mask = mask;
	return 0;
}

dma_supported() may return failiure.

static int dma_supported(struct device *dev, u64 mask)
{
	const struct dma_map_ops *ops = get_dma_ops(dev);

	/*
	 * ->dma_supported sets the bypass flag, so we must always call
	 * into the method here unless the device is truly direct mapped.
	 */
	if (!ops)
		return dma_direct_supported(dev, mask);
	if (!ops->dma_supported)
		return 1;
	return ops->dma_supported(dev, mask);
                    ^^^^^^
			DMA driver or IOMMU driver may return failure.
}

 
Frank

> 
> CJ
> 
> 
> [1]: https://lkml.org/lkml/2021/6/7/398
> 
> > +			return -EIO;
> > +
> >   	platform_set_drvdata(pdev, pcie);
> >   	ret = dw_pcie_ep_init(&pci->ep);
>