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

[李真能]

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
> > > characters.
> > >
> > > 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 &
> > > >    			     IORESOURCE_ROM_SHADOW));
> > > >    }
> > > > -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,
> > > > --
> > > > 2.25.1
> > > >
> > > >
> > > > No virus found
> > > > 		Checked by Hillstone Network AntiVirus