linux-kernel - Re: [QUERY] Using same ITS device ID for two PCI devices (two PCI Requestor ID)

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Date:   Mon, 20 Sep 2021 11:52:04 +0530
From:   Kishon Vijay Abraham I <kishon@...com>
To:     Marc Zyngier <maz@...nel.org>
CC:     Thomas Gleixner <tglx@...utronix.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Vutla, Lokesh" <lokeshvutla@...com>,
        "Nori, Sekhar" <nsekhar@...com>
Subject: Re: [QUERY] Using same ITS device ID for two PCI devices (two PCI
 Requestor ID)

Hi Marc,

On 16/09/21 11:43 pm, Marc Zyngier wrote:
> Kishon,
> 
> On Thu, 16 Sep 2021 14:02:58 +0100,
> Kishon Vijay Abraham I <kishon@...com> wrote:
>>
>> Hi Marc, Thomas,
>>
>> TI's K3 platforms use GIT ITS for PCIe MSI/MSI-X interrupts. It uses
>> *pre_its_window* as implemented by
>> *its_enable_quirk_socionext_synquacer* in irq-gic-v3-its.c.
> 
> I see it coming... If it sounds like a car crash, if it smells like a
> car crash, it probably is a car crash...
> 
>> So PCIe controller instead of directly writing to GITS_TRANSLATER,
>> will write to a separate window and the device ID is taken from the
>> offset to which the PCIe device writes (instead of dedicated lines
>> from PCIe controller to GIC ITS). So every 4-byte register address
>> maps in this window maps to a unique ITS device id.
>>
>> All of this is already implemented in Linux Kernel
>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/irqchip/irq-gic-v3-its.c#n4645
>>
>> Now TI's AM64 has an issue in that it doesn't trigger interrupt if
>> the address in the *pre_its_window* is not aligned to 8-bytes (this
>> is due to an invalid bridge configuration in HW).
>>
>> This means there will not be interrupts for devices with PCIe
>> requestor ID, 0x1, 0x3, 0x5..., as the address in the pre-ITS window
>> would be 4 (1 << 2), 12 (3 << 2), 20 (5 << 2) respectively.
> 
> Let me get this straight: instead of using the existing infrastructure
> and propagating the RIDs to the ITS, TI decided to go our of their way
> to copy the Socionext utter madness, something that was already a
> glaring bug, and managed to add a bug of their own on top of it?

unfortunately yes.
> 
> Not only this completely breaks device isolation (the Socionext bug),
> but you can't even have an odd-numbered function generating MSIs? Good

Yes.
> thing I'm on holiday, I can have an early drink to forget...
> 
>> So in order to provide 8 byte aligned address always, I mapped the PCIe
>> requestor ID to ITS device ID such that it always provides 8-byte aligned
>> address. The DT property like below helped me achieve that.
>>
>> msi-map = <0x0 &gic_its 0x0 0x10000>;
>> msi-map-mask = <0xfffe>;
>>
>> So this would result in creating one "struct its_device" for 2 PCIe
>> devices and the pre-ITS address will be aligned to 8-bytes.
>>
>> However with this, its_alloc_device_irq() for the 2nd PCIe device is failing
>> since we create "struct its_device" with the number of interrupt vectors
>> requested by the 1st PCIe device.
>>
>> Would like to get your opinion on what would be the best way to
>> workaround this for AM64.
> 
> My preferred workaround would be to take a drill and end the life of
> this thing right now. I guess this isn't an option, so see below for
> the next best thing.
> 
>> One option would be to create a new compatible for AM64 ("ti,am64,gic-v3-its")
>> allocate a minimal number of interrupt vector while creating "struct
>> its_device". Would that be acceptable? Any other ideas?
> 
> No. Messing with the core ITS allocation isn't acceptable. If there is
> such a hack, it has to be dealt within the ITS PCI backend (I assume
> you don't have anything but PCI devices targeting the ITS, right?).

That's right!
> 
> We already have to deal with cases where the endpoints are behind
> aliasing bridges, meaning that we can't distinguish between them, and
> have to account for all the endpoints behind this bridge. You will
> have to perform something similar and compute the upper bound for
> these two devices (functions of the same device actually). See
> its_pci_msi_prepare() for the gory details.

Thanks for the hint. Let me take a stab at it.

Regards,
Kishon