Re: [edk2-devel] [edk2-platforms][PATCH 1/1] Pi4: notify VPU to load xHCI firmware before XhciDxe binds

["Ard Biesheuvel" <ard.biesheuvel@arm.com>]

On 6/1/20 11:37 AM, Andrei Warkentin wrote:
> Hi Ard,
> 
> The xHCI controller is initialized with its microcode by the VPU 
> firmware, if
> I understood correctly, synchronously. When RPI_MBOX_NOTIFY_XHCI_RESET 
> finishes, the XHCI controller is ready to go.
> 

I suppose there are no other agents that may consume the config space 
during this time, right?

> So this is not any more unsafe than any of the other mailbox calls. To 
> be honest, I think RpiFirmwareDxe should be cleaned up to replace the 
> useless spinlocks (there's no multiprocessing component) with a TPL 
> manip (I checked SynchonizationLib and it doesn't touch the TPL)
> 

The spinlock protects from re-entrancy: if an event callback routine 
(such as the one you are adding for PCI I/O protocol registration) 
attempts to do a firmware call while one is already in progress, it will 
fail.

But perhaps it is indeed better to run at TPL_NOTIFY level - that way, 
the calls will be ordered one after the other instead of one being shot 
down.


> Applying your other feedback now...
> 
> A
> ------------------------------------------------------------------------
> *From:* devel@edk2.groups.io <devel@edk2.groups.io> on behalf of Ard 
> Biesheuvel via groups.io <ard.biesheuvel=arm.com@groups.io>
> 
>> +
>> +  Status = MailboxTransaction (Cmd->BufferHead.BufferSize, RPI_MBOX_VC_CHANNEL, &Result);
>> +
> 
> So this pokes the XHCI controller via its config space? Is it safe to do
> that without raising the TPL? What happens if another config space
> access occurs concurrently?
> 
> A