On Tue, May 18, 2010 at 11:49:55AM -0500, Praveen Kalamegham wrote:
> Hello all,
>
> I'm trying to provide hot-swap support for GPU devices, specifically
> headless GPUs. The native PCIe hotplug drivers prevent this
> explicitly with the following (referencing pciehp but the same can
> be found in shpchp):
>
> int pciehp_unconfigure_device(struct slot *p_slot)
> {
> ...
> for (j = 0; j < 8; j++) {
> ...
> if ((temp->class >> 16) == PCI_BASE_CLASS_DISPLAY) {
> ctrl_err(ctrl, "Cannot remove display device %s\n",
> pci_name(temp));
> pci_dev_put(temp);
> continue;
> }
> if (temp->hdr_type == PCI_HEADER_TYPE_BRIDGE && presence) {
> pci_read_config_byte(temp, PCI_BRIDGE_CONTROL, &bctl);
> if (bctl & PCI_BRIDGE_CTL_VGA) {
> ctrl_err(ctrl,
> "Cannot remove display device %s\n",
> pci_name(temp));
> pci_dev_put(temp);
> continue;
> }
> }
> ...
> }
>
> A similar check is made in pciehp_configure_device() to prevent
> hot-adding a display device.
>
> Why is this check needed?
Because the PCI hotplug spec says that this is the "rule". No PCI
hotplugging is allowed for video devices due to the wierd BIOS issues
involved in setting up a large number of video devices.
> Is it to prevent a user from hot-removing a graphics card that is
> driving their display? If so, this seems like a heavy hammer to
> protect against a user-requested (albeit probably naive) action. I've
> tested hotplug of GPU's with a patch to remove this check and it
> appears to work without negative side effects. Is there something I'm
> missing about this check?
Read the PCI hotplug spec for more details if you are interested.
> Also, if these checks are indeed necessary I believe there may be a
> bug in pciehp_unconfigure_device(). By not setting an rc value if
> the display adapter condition is met, the function will return
> successfully without properly unconfiguring the device. Then
> remove_board() would end up powering the slot off anyway. There is
> a similar but less severe issue with pciehp_configure_device() in
> the hot-add scenario.
Patches are always welcome :)
Hope this helps,
greg k-h
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