Re: [PATCH 1/2] [RFC] ata: ahci: Respect bus DMA constraints

[Robin Murphy <robin.murphy@arm.com>]

On 2019-03-07 9:37 am, Marek Vasut wrote:
> On 3/7/19 10:32 AM, Robin Murphy wrote:
>> On 2019-03-07 12:04 am, marek.vasut@gmail.com wrote:
>>> From: Marek Vasut <marek.vasut+renesas@gmail.com>
>>>
>>> Since commit 6c2fb2ea7636 ("of/device: Set bus DMA mask as appropriate"),
>>> the upstream bus can constraint device DMA range. Respect that constraint
>>> and do not change the device DMA masks if they were already set.
>>>
>>> This is applicable e.g. on systems where the PCIe controller cannot
>>> expose
>>> the full address space range. Such a system may have a 64bit CPU with
>>> DRAM
>>> mapped both below and above the 32bit address space, yet the PCIe devices
>>> can not perform DMA directly to/from the DRAM range above the 32bit
>>> limit.
>>> Hence, for such setup to work, all the buffers must exist below the 32bit
>>> limit.
>>>
>>> Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
>>> Cc: Christoph Hellwig <hch@lst.de>
>>> Cc: Geert Uytterhoeven <geert+renesas@glider.be>
>>> Cc: Jens Axboe <axboe@fb.com>
>>> Cc: Jens Axboe <axboe@kernel.dk>
>>> Cc: Keith Busch <keith.busch@intel.com>
>>> Cc: Robin Murphy <robin.murphy@arm.com>
>>> Cc: Sagi Grimberg <sagi@grimberg.me>
>>> Cc: Wolfram Sang <wsa+renesas@sang-engineering.com>
>>> Cc: linux-renesas-soc@vger.kernel.org
>>> To: linux-ide@vger.kernel.org
>>> To: linux-nvme@lists.infradead.org
>>> ---
>>>    drivers/ata/ahci.c | 7 +++++++
>>>    1 file changed, 7 insertions(+)
>>>
>>> diff --git a/drivers/ata/ahci.c b/drivers/ata/ahci.c
>>> index 021ce46e2e57..2acce056dd8c 100644
>>> --- a/drivers/ata/ahci.c
>>> +++ b/drivers/ata/ahci.c
>>> @@ -926,6 +926,13 @@ static int ahci_configure_dma_masks(struct
>>> pci_dev *pdev, int using_dac)
>>>        if (pdev->dma_mask && pdev->dma_mask < DMA_BIT_MASK(32))
>>>            return 0;
>>>    +    /*
>>> +     * The upstream device could have applied DMA constraints already,
>>> +     * respect those and do not change the DMA masks.
>>> +     */
>>> +    if (pdev->dev.dma_mask && pdev->dev.coherent_dma_mask)
>>> +        return 0;
>>
>> At least for DT platforms, the device masks are always going to be set
>> to some initial value, which will most commonly just be the 32-bit
>> default - that should not prevent the driver from setting wider masks if
>> that's what the device really supports (in fact there are some patches
>> queued in which we're now starting to formalise that properly).
>>
>> Are you seeing a problem with a DMA API backend failing to respect
>> bus_dma_mask?
> 
> Yes, the DMA mask gets overridden here to 64bit one, which on the R-Car
> Gen3 with PCI with 32bit addressing limitation makes the AHCI driver
> fail (and NVMe driver, and xHCI PCI etc). All those PCI devices fail the
> same way because they override the DMA mask.

Right, but whoever *interprets* the device masks after the driver has 
overridden them should be taking the (smaller) bus mask into account as 
well, so the question is where is *that* not being done correctly?

Robin.