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

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Date:   Wed, 12 Jun 2019 18:18:52 +1000
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     Oded Gabbay <oded.gabbay@...il.com>
Cc:     Greg KH <gregkh@...uxfoundation.org>, linuxppc-dev@...abs.org,
        Christoph Hellwig <hch@...radead.org>,
        "Linux-Kernel@...r. Kernel. Org" <linux-kernel@...r.kernel.org>,
        Oliver OHalloran <oliveroh@....ibm.com>,
        Russell Currey <ruscur@....ibm.com>
Subject: Re: [PATCH v2 8/8] habanalabs: enable 64-bit DMA mask in POWER9

On Wed, 2019-06-12 at 09:25 +0300, Oded Gabbay wrote:
> 
> > 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.
> > 
> 
> Hmm... right know they are added to customers data-centers but what do I know ;)

Well, some customers don't know they are being sold a lemon :)

> > 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.
> 
> Thanks :)
> > 
> > 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).
> 
> MSI-X are working. The set of bit 59 doesn't apply to MSI-X
> transactions (AFAICS from the PCIe controller spec we have).

Ok.

> > 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.
> 
> Understood. In the specific system we are integrated to, that is the
> case - we have less then 48 bits. But, as you pointed out, it is not a
> generic solution but with my H/W I can't give a generic fit-all
> solution for POWER9. I'll settle for the best that I can do.
> 
> > 
> > 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.
> 
> Understood and taken care of.

Cheers,
Ben.

> > 
> > Cheers,
> > Ben.
> > 
> > 
> > 
> >