Message-ID: <58E82008.7000102@gmail.com>
Date:   Fri, 7 Apr 2017 16:26:00 -0700
From:   Frank Rowand <frowand.list@...il.com>
To:     Rob Herring <robh+dt@...nel.org>,
        Robin Murphy <Robin.Murphy@....com>,
        Sricharan <sricharan@...eaurora.org>
Cc:     "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] of: change fixup of dma-ranges size to error

On 04/07/17 10:09, Rob Herring wrote:
> + Robin, Sricharan
> 
> On Fri, Apr 7, 2017 at 12:18 AM, Frank Rowand <frowand.list@...il.com> wrote:
>> On 04/06/17 15:41, Rob Herring wrote:
>>> On Thu, Apr 6, 2017 at 1:37 PM, Frank Rowand <frowand.list@...il.com> wrote:
>>>> On 04/06/17 07:03, Rob Herring wrote:
>>>>> On Thu, Apr 6, 2017 at 1:18 AM,  <frowand.list@...il.com> wrote:
>>>>>> From: Frank Rowand <frank.rowand@...y.com>
>>>>>>
>>>>>> of_dma_get_range() has workaround code to fixup a device tree that
>>>>>> incorrectly specified a mask instead of a size for property
>>>>>> dma-ranges.  That device tree was fixed a year ago in v4.6, so
>>>>>> the workaround is no longer needed.  Leave a data validation
>>>>>> check in place, but no longer do the fixup.  Move the check
>>>>>> one level deeper in the call stack so that other possible users
>>>>>> of dma-ranges will also be protected.
>>>>>>
>>>>>> The fix to the device tree was in
>>>>>> commit c91cb9123cdd ("dtb: amd: Fix DMA ranges in device tree").
>>>>>
>>>>> NACK.
>>>>> This was by design. You can't represent a size of 2^64 or 2^32.
>>>>
>>>> I agree that being unable to represent a size of 2^32 in a u32 and
>>>> a size of 2^64 in a u64 is the underlying issue.
>>>>
>>>> But the code to convert a mask to a size is _not_ design, it is a
>>>> hack that temporarily worked around a device tree that did not follow
>>>> the dma-ranges binding in the ePAPR.
>>>
>>> Since when is (2^64 - 1) not a size. It's a perfectly valid size in
>>
>> I did not say (2^64 -1) is not a size.
>>
>> I said that the existing code has a hack that converts what is perceived
>> to be a mask into a size.  The existing code is:
>>
>> @@ 110,21 @@ void of_dma_configure(struct device *dev, struct device_node *np)
>>                 size = dev->coherent_dma_mask + 1;
>>         } else {
>>                 offset = PFN_DOWN(paddr - dma_addr);
>>
>>                 /*
>>                  * Add a work around to treat the size as mask + 1 in case
>>                  * it is defined in DT as a mask.
>>                  */
>>                 if (size & 1) {
>>                         dev_warn(dev, "Invalid size 0x%llx for dma-range\n",
>>                                  size);
>>                         size = size + 1;
>>                 }
>>
>>                 if (!size) {
>>                         dev_err(dev, "Adjusted size 0x%llx invalid\n", size);
>>                         return;
>>                 }
>>                 dev_dbg(dev, "dma_pfn_offset(%#08lx)\n", offset);
>>         }
>>
>> Note the comment that says "in case it is defined in DT as a mask."
>>
>> And as you stated in a review comment is 2015: "Also, we need a WARN
>> here so DTs get fixed."
> 
> Indeed. I agree that "let me put a mask in the DT so Linux (at some
> version) works" is wrong. I still think (2^32 - 1) and (2^64 - 1)
> should be allowed to avoid growing #size-cells and because
> #size-cells=3 doesn't work.
> 
>>> DT. And there's probably not a system in the world that needs access
>>> to that last byte. Is it completely accurate description if we
>>> subtract off 1? No, but it is still a valid range (so would be
>>> subtracting 12345).
>>>
>>>> That device tree was corrected a year ago to provide a size instead of
>>>> a mask.
>>>
>>> You are letting Linux implementation details influence your DT
>>> thinking. DT is much more flexible in that it supports a base address
>>> and size (and multiple of them) while Linux can only deal with a
>>> single address mask. If Linux dealt with base + size, then we wouldn't
>>
>> No.  of_dma_get_range() returns two addresses and a size from the
>> dma-ranges property, just as it is defined in the spec.
>>
>> of_dma_configure() then interprets an odd size as meaning that the
>> device tree incorrectly contains a mask, and then converts that mask
>> to a size by adding one to it.  Linux is _still_ using address and
>> size at this point.  It does _not_ convert this size into a mask,
>> but instead passes size on into arch_setup_dma_ops().
> 
> It doesn't really matter where in the implementation, but at some
> point we end up with only a mask in Linux was my point.
> 
>> The proposed patch is to quit accepting a mask as valid data in
>> dma-ranges.
>>
>>
>>> be having this conversation. As long as Linux only deals with masks,
>>> we're going to have to have some sort of work-around to deal with
>>> them.
>>>
>>>>> Well, technically you can for the latter, but then you have to grow
>>>>> #size-cells to 2 for an otherwise all 32-bit system which seems kind
>>>>> of pointless and wasteful. You could further restrict this to only
>>>>> allow ~0 and not just any case with bit 0 set.
>>>>>
>>>>> I'm pretty sure AMD is not the only system. There were 32-bit systems too.
>>>>
>>>> I examined all instances of property dma-ranges in in tree dts files in
>>>> Linux 4.11-rc1.  There are none that incorrectly specify mask instead of
>>>> size.
>>>
>>> Okay, but there are ones for ranges at least. See ecx-2000.dts.
>>
>> The patch does not impact the ranges property.  It only impacts the
>> dma-ranges property.
> 
> Yes, I know. I'm only pointing out we have other cases of size=~0 to
> avoid growing #size-cells.
> 
>>>> #size-cells only changes to 2 for the dma-ranges property and the ranges
>>>> property when size is 2^32, so that is a very small amount of space.
>>>>
>>>> The patch does not allow for a size of 2^64.  If a system requires a
>>>> size of 2^64 then the type of size needs to increase to be larger
>>>> than a u64.  If you would like for the code to be defensive and
>>>> detect a device tree providing a size of 2^64 then I can add a
>>>> check to of_dma_get_range() to return -EINVAL if #size-cells > 2.
>>>> When that error triggers, the type of size can be changed.
>>>
>>> #size-cells > 2 is completely broken for anything but PCI. I doubt it
>>
>> Yes, that is what I said.  The current code does not support #size-cells > 2
>> for dma-ranges.
> 
> It's not just dma-ranges. It's everywhere with reg and ranges and any
> code that parses those too. If someone needs to truly specify sizes of
> 2^64 in DT (for reg, ranges, or dma-ranges), they are SOL.
> 
>> #size-cells > 2 for dma-ranges will lead to a problem in
>> of_dma_get_range(), which stuffs the value of the size into a u64.
>> Clearly, a 3 cell size will not fit into a u64.
>>
>>
>>> is easily fixed without some special casing (i.e. a different hack)
>>> until we have 128-bit support. I hope to retire before we need to
>>> support that.
>>>
>>> Rob
>>>
>>
>> Can we get back to the basic premise of the proposed patch?
>>
>> The current code in of_dma_configure() contains a hack that allows the
>> dma-ranges property to specify a mask instead of a size.  The binding
>> in the specification allows a size and does not allow a mask.
>>
>> The hack was added to account for one or more dts files that did not
>> follow the specification.  In the mail list discussion of the hack
>> you said "Also, we need a WARN here so DTs get fixed."
>>
>> The hack was first present in Linux 4.1.  The only in-tree dts that
>> incorrectly contained a mask instead of a size in dma-ranges was
>> arch/arm64/boot/dts/amd/amd-seattle-soc.dtsi
>>
>> That .dtsi was fixed by
>> commit c91cb9123cdd ("dtb: amd: Fix DMA ranges in device tree")
>> The fix was present in Linux 4.6, May 15, 2016.
>>
>> I would like to remove the hack.  I think that enough time has
>> elapsed to allow this change.
> 
> If we have no cases of what I'm concerned about, then removing it is
> fine. Is this a dependency for iommu series? Doesn't look like it to
> me.

This patch is a replacement for patch 03/12 in the iommu series.  I
think that patch 03/12 of the iommu series could be dropped and my
patch could be applied independently of the iommu series.

There is likely a conflict between my patch and patch 06/12 of the
iommu series because in my patch the first line of the patch chunk
of drivers/of/device.c includes a line that is changed in 06/12
of the iommu series.  If this is the case then the iommu series
should take precedence over my patch (and I should subsequently
fixup my patch).

-Frank

> 
> Rob
> 

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