[PATCH kernel] powerpc/dma: Fix dma_map_ops::get_required_mask

[Alexey Kardashevskiy]

On 15/09/2020 16:50, Christoph Hellwig wrote:
> On Wed, Sep 09, 2020 at 07:36:04PM +1000, Alexey Kardashevskiy wrote:
>> I want dma_get_required_mask() to return the bigger mask always.
>>
>> Now it depends on (in dma_alloc_direct()):
>> 1. dev->dma_ops_bypass: set via pci_set_(coherent_)dma_mask();
>> 2. dev->coherent_dma_mask - the same;
>> 3. dev->bus_dma_limit - usually not set at all.
>>
>> So until we set the mask, dma_get_required_mask() returns smaller mask.
>> So aacraid and likes (which calls dma_get_required_mask() before setting
>> it) will remain prone for breaks.
> 
> Well, the original intent of dma_get_required_mask is to return the
> mask that the driver then uses to figure out what to set, so what aacraid
> does fits that use case. 

What was the original intent exactly? The driver asks for the minimum or
maximum DMA mask the platform supports?

As for now, we (ppc64/powernv) can do:
1. bypass (==64bit)
2. a DMA window which used to be limited by 2GB but not anymore.

I can understand if the driver asked for required mask in expectation to
receive "less or equal than 32bit" and "more than 32 bit" and choose.
And this probably was the intent as at the time when the bug was
introduced, the window was always smaller than 4GB.

But today the window is bigger than than (44 bits now, or a similar
value, depends on max page order) so the returned mask is >32. Which
still enables that DAC in aacraid but I suspect this is accidental.


> Of course that idea is pretty bogus for
> PCIe devices.

Why? From the PHB side, there are windows. From the device side, there
are many crippled devices, like, no GPU I saw in last years supported
more than 48bit.


> I suspect the right fix is to just not query dma_get_required_mask for
> PCIe devices in aacraid (and other drivers that do something similar).

May be, if you write nice and big comment next to
dma_get_required_mask() explaining exactly what it does, then I will
realize I am getting this all wrong and we will move to fixing the
drivers :)


-- 
Alexey