netdev - Re: [PATCH v4 virtio 01/10] virtio: allow caller to override device id and DMA mask

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Message-ID: <ZE/QS0lnUvxFacjf@corigine.com>
Date:   Mon, 1 May 2023 16:44:27 +0200
From:   Simon Horman <simon.horman@...igine.com>
To:     Shannon Nelson <shannon.nelson@....com>
Cc:     jasowang@...hat.com, mst@...hat.com,
        virtualization@...ts.linux-foundation.org, brett.creeley@....com,
        netdev@...r.kernel.org, drivers@...sando.io
Subject: Re: [PATCH v4 virtio 01/10] virtio: allow caller to override device
 id and DMA mask

On Tue, Apr 25, 2023 at 02:25:53PM -0700, Shannon Nelson wrote:
> To add a bit of flexibility with various virtio based devices, allow
> the caller to specify a different device id and DMA mask.  This adds
> fields to struct virtio_pci_modern_device to specify an override device
> id check and a DMA mask.
> 
> int (*device_id_check)(struct pci_dev *pdev);
> 	If defined by the driver, this function will be called to check
> 	that the PCI device is the vendor's expected device, and will
> 	return the found device id to be stored in mdev->id.device.
> 	This allows vendors with alternative vendor device ids to use
> 	this library on their own device BAR.
> 
> u64 dma_mask;
> 	If defined by the driver, this mask will be used in a call to
> 	dma_set_mask_and_coherent() instead of the traditional
> 	DMA_BIT_MASK(64).  This allows limiting the DMA space on
> 	vendor devices with address limitations.

Hi Shannon,

I don't feel strongly about this, but as there are two new features,
perhaps it would appropriate to have two patches.

> Signed-off-by: Shannon Nelson <shannon.nelson@....com>
> ---
>  drivers/virtio/virtio_pci_modern_dev.c | 37 +++++++++++++++++---------
>  include/linux/virtio_pci_modern.h      |  6 +++++
>  2 files changed, 31 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_pci_modern_dev.c b/drivers/virtio/virtio_pci_modern_dev.c
> index 869cb46bef96..1f2db76e8f91 100644
> --- a/drivers/virtio/virtio_pci_modern_dev.c
> +++ b/drivers/virtio/virtio_pci_modern_dev.c
> @@ -218,21 +218,29 @@ int vp_modern_probe(struct virtio_pci_modern_device *mdev)
>  	int err, common, isr, notify, device;
>  	u32 notify_length;
>  	u32 notify_offset;
> +	int devid;
>  
>  	check_offsets();
>  
> -	/* We only own devices >= 0x1000 and <= 0x107f: leave the rest. */
> -	if (pci_dev->device < 0x1000 || pci_dev->device > 0x107f)
> -		return -ENODEV;
> -
> -	if (pci_dev->device < 0x1040) {
> -		/* Transitional devices: use the PCI subsystem device id as
> -		 * virtio device id, same as legacy driver always did.
> -		 */
> -		mdev->id.device = pci_dev->subsystem_device;
> +	if (mdev->device_id_check) {
> +		devid = mdev->device_id_check(pci_dev);
> +		if (devid < 0)
> +			return devid;
> +		mdev->id.device = devid;
>  	} else {
> -		/* Modern devices: simply use PCI device id, but start from 0x1040. */
> -		mdev->id.device = pci_dev->device - 0x1040;
> +		/* We only own devices >= 0x1000 and <= 0x107f: leave the rest. */
> +		if (pci_dev->device < 0x1000 || pci_dev->device > 0x107f)
> +			return -ENODEV;
> +
> +		if (pci_dev->device < 0x1040) {
> +			/* Transitional devices: use the PCI subsystem device id as
> +			 * virtio device id, same as legacy driver always did.
> +			 */
> +			mdev->id.device = pci_dev->subsystem_device;
> +		} else {
> +			/* Modern devices: simply use PCI device id, but start from 0x1040. */
> +			mdev->id.device = pci_dev->device - 0x1040;
> +		}
>  	}
>  	mdev->id.vendor = pci_dev->subsystem_vendor;
>  

The diff above is verbose, but looks good to me :)

> @@ -260,7 +268,12 @@ int vp_modern_probe(struct virtio_pci_modern_device *mdev)
>  		return -EINVAL;
>  	}
>  
> -	err = dma_set_mask_and_coherent(&pci_dev->dev, DMA_BIT_MASK(64));
> +	if (mdev->dma_mask)
> +		err = dma_set_mask_and_coherent(&pci_dev->dev,
> +						mdev->dma_mask);
> +	else
> +		err = dma_set_mask_and_coherent(&pci_dev->dev,
> +						DMA_BIT_MASK(64));

Maybe it is nicer to avoid duplicating the function call, something like
this:

	u64 dma_mask;
	...

	dma_mask = mdev->dma_mask ? : DMA_BIT_MASK(64);
	err = dma_set_mask_and_coherent(&pci_dev->dev, dma_mask);

or, without a local variable.

	err = dma_set_mask_and_coherent(&pci_dev->dev,
					mdev->dma_mask ? : DMA_BIT_MASK(64));

>  	if (err)
>  		err = dma_set_mask_and_coherent(&pci_dev->dev,
>  						DMA_BIT_MASK(32));
> diff --git a/include/linux/virtio_pci_modern.h b/include/linux/virtio_pci_modern.h
> index c4eeb79b0139..067ac1d789bc 100644
> --- a/include/linux/virtio_pci_modern.h
> +++ b/include/linux/virtio_pci_modern.h

Maybe it would be good to add kdoc for struct virtio_pci_modern_device
at some point.

> @@ -38,6 +38,12 @@ struct virtio_pci_modern_device {
>  	int modern_bars;
>  
>  	struct virtio_device_id id;
> +
> +	/* optional check for vendor virtio device, returns dev_id or -ERRNO */
> +	int (*device_id_check)(struct pci_dev *pdev);
> +
> +	/* optional mask for devices with limited DMA space */
> +	u64 dma_mask;
>  };
>  
>  /*
> -- 
> 2.17.1
>