linux-kernel - Re: [PATCH v9 2/3] PCI: Add tango PCIe host bridge support

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Date:   Wed, 5 Jul 2017 01:59:55 +0200
From:   Mason <slash.tmp@...e.fr>
To:     Russell King - ARM Linux <linux@...linux.org.uk>,
        Bjorn Helgaas <helgaas@...nel.org>
Cc:     Marc Gonzalez <marc_gonzalez@...madesigns.com>,
        Mark Rutland <mark.rutland@....com>,
        Marc Zyngier <marc.zyngier@....com>,
        linux-pci <linux-pci@...r.kernel.org>,
        Thibaud Cornic <thibaud_cornic@...madesigns.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v9 2/3] PCI: Add tango PCIe host bridge support

On 03/07/2017 20:11, Russell King - ARM Linux wrote:

> I don't think there's an easy solution to this problem - and I'm not
> sure that stop_machine() can be made to work in this path (which
> needs a process context).  I have a suspicion that the Sigma Designs
> PCI implementation is just soo insane that it's never going to work
> reliably in a multi-SoC kernel without introducing severe performance
> issues for everyone else.

If I remember correctly, this is the second HW block from
tango that has been deemed "too insane for Linux".

The first one was the DMA engine, which doesn't interrupt
when a transfer is done, but when a new transfer may be
programmed. (Though there is a simple work-around for
this one, if we give up command pipelining.)

Do larger SoC vendors have HW devs working closely with
Linux devs, to avoid these design bloopers?

Regards.