Re: [PATCH] PCI/sysfs: add write attribute for boot_vga

From: 李真能
Date: Tue Sep 28 2021 - 21:46:41 EST

在 2021/9/29 上午7:37, Bjorn Helgaas 写道:
On Mon, Sep 27, 2021 at 11:45:59AM +0800, 李真能 wrote:
在 2021/9/27 上午4:20, Bjorn Helgaas 写道:
On Sun, Sep 26, 2021 at 03:15:39PM +0800, Zhenneng Li wrote:
Add writing attribute for boot_vga sys node,
so we can config default video display
output dynamically when there are two video
cards on a machine.

Xorg server will determine running on which
video card based on boot_vga node's value.
When you repost this, please take a look at the git commit log history
and make yours similar. Specifically, the subject should start with a
capital letter, and the body should be rewrapped to fill 75

Please contrast this with the existing VGA arbiter. See
Documentation/gpu/vgaarbiter.rst. It sounds like this may overlap
with the VGA arbiter functionality, so this should explain why we need
both and how they interact.
"Some "legacy" VGA devices implemented on PCI typically have the same
hard-decoded addresses as they did on ISA. When multiple PCI devices are
accessed at same time they need some kind of coordination. ", this is the
explain of config VGA_ARB, that is to say, some legacy vga devices need use
the same pci bus address, if user app(such as xorg) want access card A, but
card A and card B have same bus address,  then VGA agaarbiter will determine
will card to be accessed.
Yes. I think the arbiter also provides an interface for controlling
the routing of these legacy resources.

Your patch changes the kernel's idea of the default VGA device, but
doesn't affect the resource routing, AFAICT.

And xorg will read boot_vga to determine which graphics card is the primary
graphics output device.
Doesn't xorg also have its own mechanism for selecting which graphics
device to use?

Is the point here that you want to write the sysfs file to select the
device instead of changing the xorg configuration? If it's possible
to configure xorg directly to use different devices, my inclination
would be to use that instead of doing it via sysfs.
Thanks for reminding, Xorg has the config option "PrimaryGPU", it has the same func as boot_vga.

That is the difference about boot_vga and vgaarbiter.

Signed-off-by: Zhenneng Li <lizhenneng@xxxxxxxxxx>
drivers/pci/pci-sysfs.c | 24 +++++++++++++++++++++++-
1 file changed, 23 insertions(+), 1 deletion(-)

diff --git a/drivers/pci/pci-sysfs.c b/drivers/pci/pci-sysfs.c
index 7bbf2673c7f2..a6ba19ce7adb 100644
--- a/drivers/pci/pci-sysfs.c
+++ b/drivers/pci/pci-sysfs.c
@@ -664,7 +664,29 @@ static ssize_t boot_vga_show(struct device *dev, struct device_attribute *attr,
!!(pdev->resource[PCI_ROM_RESOURCE].flags &
-static DEVICE_ATTR_RO(boot_vga);
+static ssize_t boot_vga_store(struct device *dev, struct device_attribute *attr,
+ const char *buf, size_t count)
+ unsigned long val;
+ struct pci_dev *pdev = to_pci_dev(dev);
+ struct pci_dev *vga_dev = vga_default_device();
+ if (kstrtoul(buf, 0, &val) < 0)
+ return -EINVAL;
+ if (val != 1)
+ return -EINVAL;
+ if (!capable(CAP_SYS_ADMIN))
+ return -EPERM;
+ if (pdev != vga_dev)
+ vga_set_default_device(pdev);
+ return count;
+static DEVICE_ATTR_RW(boot_vga);
static ssize_t pci_read_config(struct file *filp, struct kobject *kobj,
struct bin_attribute *bin_attr, char *buf,

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