linux-kernel - Re: [PATCH v2 8/8] habanalabs: enable 64-bit DMA mask in POWER9

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Message-ID: <CAOSf1CFcYnhKf0EJkm+E5uHg4e=QYGe+vAUz_TjX-v6UsmNtFA@mail.gmail.com>
Date:   Wed, 12 Jun 2019 15:45:17 +1000
From:   "Oliver O'Halloran" <oohall@...il.com>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:     Oded Gabbay <oded.gabbay@...il.com>,
        Greg KH <gregkh@...uxfoundation.org>, linuxppc-dev@...abs.org,
        Christoph Hellwig <hch@...radead.org>,
        Russell Currey <ruscur@....ibm.com>,
        "Linux-Kernel@...r. Kernel. Org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 8/8] habanalabs: enable 64-bit DMA mask in POWER9

On Wed, Jun 12, 2019 at 8:54 AM Benjamin Herrenschmidt
<benh@...nel.crashing.org> wrote:
>
> On Tue, 2019-06-11 at 20:22 +0300, Oded Gabbay wrote:
> >
> > > So, to summarize:
> > > If I call pci_set_dma_mask with 48, then it fails on POWER9. However,
> > > in runtime, I don't know if its POWER9 or not, so upon failure I will
> > > call it again with 32, which makes our device pretty much unusable.
> > > If I call pci_set_dma_mask with 64, and do the dedicated configuration
> > > in Goya's PCIe controller, then it won't work on x86-64, because bit
> > > 59 will be set and the host won't like it (I checked it). In addition,
> > > I might get addresses above 50 bits, which my device can't generate.
> > >
> > > I hope this makes things more clear. Now, please explain to me how I
> > > can call pci_set_dma_mask without any regard to whether I run on
> > > x86-64 or POWER9, considering what I wrote above ?
> > >
> > > Thanks,
> > > Oded
> >
> > Adding ppc mailing list.
>
> You can't. Your device is broken. Devices that don't support DMAing to
> the full 64-bit deserve to be added to the trash pile.
>
> As a result, getting it to work will require hacks. Some GPUs have
> similar issues and require similar hacks, it's unfortunate.
>
> Added a couple of guys on CC who might be able to help get those hacks
> right.

> It's still very fishy .. the idea is to detect the case where setting a
> 64-bit mask will give your system memory mapped at a fixed high address
> (1 << 59 in our case) and program that in your chip in the "Fixed high
> bits" register that you seem to have (also make sure it doesn't affect
> MSIs or it will break them).

Judging from the patch (https://lkml.org/lkml/2019/6/11/59) this is
what they're doing.

Also, are you sure about the MSI thing? The IODA3 spec says the only
important bits for a 64bit MSI are bits 61:60 (to hit the window) and
the lower bits that determine what IVE to use. Everything in between
is ignored so ORing in bit 59 shouldn't break anything.

> This will only work as long as all of the system memory can be
> addressed at an offset from that fixed address that itself fits your
> device addressing capabilities (50 bits in this case). It may or may
> not be the case but there's no way to check since the DMA mask logic
> won't really apply.
>
> You might want to consider fixing your HW in the next iteration... This
> is going to bite you when x86 increases the max physical memory for
> example, or on other architectures.

Yes, do this. The easiest way to avoid this sort of wierd hack is to
just design the PCIe interface to the spec in the first place.