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

[李真能 <lizhenneng@kylinos.cn>]

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.

And xorg will read boot_vga to determine which graphics card is the 
primary graphics output device.

That is the difference about boot_vga and vgaarbiter.



>
>> Signed-off-by: Zhenneng Li <lizhenneng@kylinos.cn>
>> ---
>>   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
>>
>>
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