linux-kernel - Re: [PATCH v3 0/3] uio: add MSI/MSI-X support to uio_pci

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Date:	Tue, 6 Oct 2015 17:30:54 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Vlad Zolotarov <vladz@...udius-systems.com>
Cc:	linux-kernel@...r.kernel.org, hjk@...sjkoch.de, corbet@....net,
	gregkh@...uxfoundation.org, bruce.richardson@...el.com,
	avi@...udius-systems.com, gleb@...udius-systems.com,
	stephen@...workplumber.org, alexander.duyck@...il.com
Subject: Re: [PATCH v3 0/3] uio: add MSI/MSI-X support to uio_pci_generic
 driver

On Tue, Oct 06, 2015 at 11:37:59AM +0300, Vlad Zolotarov wrote:
> Bus mastering is easily enabled from the user space (taken from DPDK code):
> 
> static int
> pci_uio_set_bus_master(int dev_fd)
> {
> 	uint16_t reg;
> 	int ret;
> 
> 	ret = pread(dev_fd, &reg, sizeof(reg), PCI_COMMAND);
> 	if (ret != sizeof(reg)) {
> 		RTE_LOG(ERR, EAL,
> 			"Cannot read command from PCI config space!\n");
> 		return -1;
> 	}
> 
> 	/* return if bus mastering is already on */
> 	if (reg & PCI_COMMAND_MASTER)
> 		return 0;
> 
> 	reg |= PCI_COMMAND_MASTER;
> 
> 	ret = pwrite(dev_fd, &reg, sizeof(reg), PCI_COMMAND);
> 	if (ret != sizeof(reg)) {
> 		RTE_LOG(ERR, EAL,
> 			"Cannot write command to PCI config space!\n");
> 		return -1;
> 	}
> 
> 	return 0;
> }
> 
> So, this is a non-issue. ;)

There might be valid reasons for DPDK to do this, e.g. if using VFIO.

I'm guessing it doesn't enable MSI though, does it?

-- 
MST
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