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References: <1247576250-16274-1-git-send-email-tiago.vignatti@nokia.com>
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Date: Wed, 15 Jul 2009 14:43:38 +1000
Message-ID: <21d7e9970907142143y5e289ffdld0cfd73ef0106953@mail.gmail.com>
Subject: Re: [PATCH 1/2] vga: implements VGA arbitration on Linux
From: Dave Airlie <airlied@gmail.com>
To: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Tiago Vignatti <tiago.vignatti@nokia.com>,
       Jesse Barnes <jbarnes@virtuousgeek.org>,
       Dave Airlie <airlied@redhat.com>, linux-pci@vger.kernel.org,
       linux-kernel@vger.kernel.org
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On Wed, Jul 15, 2009 at 12:35 AM, Alan Cox<alan@lxorguk.ukuu.org.uk> wrote:
>> +#ifndef __ARCH_HAS_VGA_ENABLE_RESOURCES
>> +static inline void vga_enable_resources(struct pci_dev *pdev,
>> + ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? unsigned int rsrc)
>> +{
>> + ? ? struct pci_bus *bus;
>> + ? ? struct pci_dev *bridge;
>> + ? ? u16 cmd;
>> +
>> +#ifdef DEBUG
>> + ? ? printk(KERN_DEBUG "%s\n", __func__);
>> +#endif
>> + ? ? pci_read_config_word(pdev, PCI_COMMAND, &cmd);
>> + ? ? if (rsrc & (VGA_RSRC_LEGACY_IO | VGA_RSRC_NORMAL_IO))
>> + ? ? ? ? ? ? cmd |= PCI_COMMAND_IO;
>> + ? ? if (rsrc & (VGA_RSRC_LEGACY_MEM | VGA_RSRC_NORMAL_MEM))
>> + ? ? ? ? ? ? cmd |= PCI_COMMAND_MEMORY;
>> + ? ? pci_write_config_word(pdev, PCI_COMMAND, cmd);
>
> Locking question - what locks this lot against hotplug also touching
> bridge settings ?
>
>> +
>> + ? ? if (!(rsrc & VGA_RSRC_LEGACY_MASK))
>> + ? ? ? ? ? ? return;
>> +
>> + ? ? bus = pdev->bus;
>> + ? ? while (bus) {
>> + ? ? ? ? ? ? bridge = bus->self;
>> + ? ? ? ? ? ? if (bridge) {
>> + ? ? ? ? ? ? ? ? ? ? pci_read_config_word(bridge, PCI_BRIDGE_CONTROL,
>> + ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?&cmd);
>> + ? ? ? ? ? ? ? ? ? ? if (!(cmd & PCI_BRIDGE_CTL_VGA)) {
>> + ? ? ? ? ? ? ? ? ? ? ? ? ? ? cmd |= PCI_BRIDGE_CTL_VGA;
>> + ? ? ? ? ? ? ? ? ? ? ? ? ? ? pci_write_config_word(bridge,
>> + ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? PCI_BRIDGE_CONTROL,
>> + ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? cmd);
>> + ? ? ? ? ? ? ? ? ? ? }
>> + ? ? ? ? ? ? }
>> + ? ? ? ? ? ? bus = bus->parent;
>> + ? ? }
>> +}
>> +#endif
>
>
>> + ? ? /* The one who calls us should check for this, but lets be sure... */
>> + ? ? if (pdev == NULL)
>> + ? ? ? ? ? ? pdev = vga_default_device();
>
> What if the BIOS provided device was hot unplugged ?
>
>> + ? ? ? ? ? ? conflict = __vga_tryget(vgadev, rsrc);
>> + ? ? ? ? ? ? spin_unlock_irqrestore(&vga_lock, flags);
>> + ? ? ? ? ? ? if (conflict == NULL)
>> + ? ? ? ? ? ? ? ? ? ? break;
>> +
>> +
>> + ? ? ? ? ? ? /* We have a conflict, we wait until somebody kicks the
>> + ? ? ? ? ? ? ?* work queue. Currently we have one work queue that we
>
> If two drivers own half the resources and both are waiting for the rest
> what handles the deadlock

I'm not sure we should care about resource locking granularity below,
I have all resources, I have no resources, I don't think we gain anything
by it, since really we want to avoid using vga arb as much as we can.

Tiago, the other issue I noticed when using this is we need to provide
 a hook for drm drivers that use an irq to have the irq disabled around
the mem/io space disables otherwise they can get an irq with no mem/io
and boom all fall down.

Old X userspace RAC never worried about interrupts and assumed the
GPU wasn't using one which was true in the 80s.

Dave.
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