Re: [PATCH] ata: ahci_platform: add 32-bit quirk for dwc-ahci

[Roger Quadros <rogerq@ti.com>]

On 12/02/2020 13:43, Robin Murphy wrote:
> On 2020-02-12 11:32 am, Hans de Goede wrote:
>> Hi,
>>
>> On 2/12/20 12:01 PM, Roger Quadros wrote:
>>> Hi,
>>>
>>> On 06/02/2020 13:50, Hans de Goede wrote:
>>>> Hi,
>>>>
>>>> On 2/6/20 12:17 PM, Roger Quadros wrote:
>>>>> On TI Platforms using LPAE, SATA breaks with 64-bit DMA.
>>>>> Restrict it to 32-bit.
>>>>>
>>>>> Cc: stable@vger.kernel.org
>>>>> Signed-off-by: Roger Quadros <rogerq@ti.com>
>>>>> ---
>>>>>   drivers/ata/ahci_platform.c | 3 +++
>>>>>   1 file changed, 3 insertions(+)
>>>>>
>>>>> diff --git a/drivers/ata/ahci_platform.c b/drivers/ata/ahci_platform.c
>>>>> index 3aab2e3d57f3..b925dc54cfa5 100644
>>>>> --- a/drivers/ata/ahci_platform.c
>>>>> +++ b/drivers/ata/ahci_platform.c
>>>>> @@ -62,6 +62,9 @@ static int ahci_probe(struct platform_device *pdev)
>>>>>       if (of_device_is_compatible(dev->of_node, "hisilicon,hisi-ahci"))
>>>>>           hpriv->flags |= AHCI_HFLAG_NO_FBS | AHCI_HFLAG_NO_NCQ;
>>>>> +    if (of_device_is_compatible(dev->of_node, "snps,dwc-ahci"))
>>>>> +        hpriv->flags |= AHCI_HFLAG_32BIT_ONLY;
>>>>> +
>>>>
>>>> The "snps,dwc-ahci" is a generic (non TI specific) compatible which
>>>> is e.g. also used on some exynos devices. So using that to key the
>>>> setting of the 32 bit flag seems wrong to me.
>>>>
>>>> IMHO it would be better to introduce a TI specific compatible
>>>> and use that to match on instead (and also adjust the dts files
>>>> accordingly).
>>>
>>> Thinking further on this I think it is a bad idea to add a special
>>> binding because the IP is not different. It is just that it is
>>> wired differently on the TI SoC so DMA range is limited.
>>>
>>> IMO the proper solution is to have the right dma-ranges property in the
>>> device tree. However, SATA platform driver is doing the wrong thing
>>> by overriding the dma masks.
>>> i.e. in ahci_platform_init_host() in libahci_platform.c >
>>>          if (hpriv->cap & HOST_CAP_64) {
>>>                  rc = dma_coerce_mask_and_coherent(dev, DMA_BIT_MASK(64));
>>>                  if (rc) {
>>>                          rc = dma_coerce_mask_and_coherent(dev,
>>> DMA_BIT_MASK(32));
>>>                          if (rc) {
>>>                                  dev_err(dev, "Failed to enable 64-bit DMA.\n");
>>>                                  return rc;
>>>                          }
>>>                          dev_warn(dev, "Enable 32-bit DMA instead of 64-bit.\n");
>>>                  }
>>>          }
>>>
>>> This should be removed. Do you agree?
>>
>> I agree with you in principal, but I'm afraid this might cause regressions for
>> existing hardware. We only do this if the host has set the CAP_64 flag,
>> this code is quite old, it comes from the following commit:
>>
>> ###
>>  From cc7a9e27562cd78a1dc885504086fab24addce40 Mon Sep 17 00:00:00 2001
>> From: Suravee Suthikulpanit <Suravee.Suthikulpanit@amd.com>
>> Date: Thu, 12 Jun 2014 12:40:23 -0500
>> Subject: [PATCH v3] ahci: Check and set 64-bit DMA mask for platform AHCI driver
>>
>> The current platform AHCI driver does not set the dma_mask correctly
>> for 64-bit DMA capable AHCI controller. This patch checks the AHCI
>> capability bit and set the dma_mask and coherent_dma_mask accordingly.
>>
>> Signed-off-by: Suravee Suthikulpanit <Suravee.Suthikulpanit@amd.com>
>> Reviewed-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
>> Reviewed-by: Hans de Goede <hdegoede@redhat.com>
>> Tested-by: Hans de Goede <hdegoede@redhat.com>
>> Tested-by: Suman Tripathi <stripathi@apm.com>
>> Signed-off-by: Tejun Heo <tj@kernel.org>
>> ###
>>
>> Presumably this was added for a reason, I'm guessing this might come
>> from AMD's ARM server chips adventures, but I'm afraid that AHCI support
>> on other (ARM) SoC's has become to rely on this behavior too.
>>
>> Maybe we can add a check to see if the mask was not already set and skip
>> setting the mask in that case ?
> 
> If the device *is* inherently 64-bit capable, then setting 64-bit masks in the driver is correct - if a 64-bit IP block happens to have been integrated with only 32 address bits wired up, but the system has memory above the 32-bit boundary, then that should be described via "dma-ranges", which should then end up being used to further constrain the device masks internally to the DMA API.

I agree. In this case, it looks like DMA API is allocating memory > 32-bits
even if "dma-ranges" size and dma_bus_limit is < 32-bits so issue is with
platform DMA code.

I will continue the discussion in the other thread
https://lkml.org/lkml/2020/2/12/907

-- 
cheers,
-roger
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