linux-kernel - Re: [PATCH V2 3/4] misc: vop: simply return the saved dma address instead of virt_to

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Message-ID: <20200929102643.GC7784@infradead.org>
Date:   Tue, 29 Sep 2020 11:26:43 +0100
From:   Christoph Hellwig <hch@...radead.org>
To:     Sherry Sun <sherry.sun@....com>
Cc:     hch@...radead.org, sudeep.dutt@...el.com, ashutosh.dixit@...el.com,
        arnd@...db.de, gregkh@...uxfoundation.org, kishon@...com,
        lorenzo.pieralisi@....com, linux-kernel@...r.kernel.org,
        linux-pci@...r.kernel.org, linux-imx@....com
Subject: Re: [PATCH V2 3/4] misc: vop: simply return the saved dma address
 instead of virt_to_phys

On Tue, Sep 29, 2020 at 04:44:24PM +0800, Sherry Sun wrote:
> The device page and vring should use consistent memory which are
> allocated by dma_alloc_coherent api, when user space wants to get its
> physical address, virt_to_phys cannot be used, should simply return the
> saved device page dma address by get_dp_dma callback and the vring dma
> address saved in mic_vqconfig.

More importantly you can't just all virt_to_phys on a return value
from dma_alloc_coherent, so this needs to be folded into patch 1.

>  	if (!offset) {
> -		*pa = virt_to_phys(vpdev->hw_ops->get_dp(vpdev));
> +		if (vpdev->hw_ops->get_dp_dma)
> +			*pa = vpdev->hw_ops->get_dp_dma(vpdev);
> +		else {
> +			dev_err(vop_dev(vdev), "can't get device page physical address\n");
> +			return -EINVAL;
> +		}

I don't think we need the NULL check here.  Wouldn't it also make sense
to return the virtual and DMA address from ->get_dp instead of adding
another method?