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

[Marek Vasut <marek.vasut@gmail.com>]

On 3/7/19 10:48 AM, Robin Murphy wrote:
> 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?

Do you have a hint where I should look for that ?

-- 
Best regards,
Marek Vasut