Re: [BUG] ls1046a: eDMA does not transfer data from I2C

[Sean Anderson]

On 9/20/22 11:44 AM, Sean Anderson wrote:
> 
> 
> On 9/20/22 11:24 AM, Sean Anderson wrote:
>> 
>> 
>> On 9/20/22 6:07 AM, Robin Murphy wrote:
>>> On 2022-09-19 23:24, Sean Anderson wrote:
>>>> Hi all,
>>>>
>>>> I discovered a bug in either imx_i2c or fsl-edma on the LS1046A where no
>>>> data is read in i2c_imx_dma_read except for the last two bytes (which
>>>> are not read using DMA). This is perhaps best illustrated with the
>>>> following example:
>>>>
>>>> # hexdump -C /sys/bus/nvmem/devices/0-00540/nvmem
>>>> [  308.914884] i2c i2c-0: ffff000809380000 0x0000000889380000 0x00000000f5401000 ffff000075401000
>>>> [  308.923529] src= 2180004 dst=f5401000 attr=   0 soff=   0 nbytes=1 slast=       0
>>>> [  308.923529] citer=  7e biter=  7e doff=   1 dlast_sga=       0
>>>> [  308.923529] major_int=1 disable_req=1 enable_sg=0
>>>> [  308.942113] fsl-edma 2c00000.edma: vchan 000000001b4371fc: txd 00000000d9dd26c5[4]: submitted
>>>> [  308.974049] fsl-edma 2c00000.edma: txd 00000000d9dd26c5[4]: marked complete
>>>> [  308.981339] i2c i2c-0: ffff000809380000 = [2e 2e 2f 2e 2e 2f 2e 2e 2f 64 65 76 69 63 65 73 2f 70 6c 61 74 66 6f 72 6d 2f 73 6f 63 2f 32 31 38 30 30 30 30 2e 69 32 63 2f 69 32 63 2d 30 2f 30 2d 30 30 35 34 2f 30 2d 30 30 35 34 30 00 00]
>>>> [  309.002226] i2c i2c-0: ffff000075401000 = [2e 2e 2f 2e 2e 2f 2e 2e 2f 64 65 76 69 63 65 73 2f 70 6c 61 74 66 6f 72 6d 2f 73 6f 63 2f 32 31 38 30 30 30 30 2e 69 32 63 2f 69 32 63 2d 30 2f 30 2d 30 30 35 34 2f 30 2d 30 30 35 34 30 00 00]
>>>> [  309.024649] i2c i2c-0: ffff000809380080 0x0000000889380080 0x00000000f5401800 ffff000075401800
>>>> [  309.033270] src= 2180004 dst=f5401800 attr=   0 soff=   0 nbytes=1 slast=       0
>>>> [  309.033270] citer=  7e biter=  7e doff=   1 dlast_sga=       0
>>>> [  309.033270] major_int=1 disable_req=1 enable_sg=0
>>>> [  309.051633] fsl-edma 2c00000.edma: vchan 000000001b4371fc: txd 00000000d9dd26c5[5]: submitted
>>>> [  309.083526] fsl-edma 2c00000.edma: txd 00000000d9dd26c5[5]: marked complete
>>>> [  309.090807] i2c i2c-0: ffff000809380080 = [00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00]
>>>> [  309.111694] i2c i2c-0: ffff000075401800 = [00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00]
>>>> 00000000  2e 2e 2f 2e 2e 2f 2e 2e  2f 64 65 76 69 63 65 73  |../../../devices|
>>>> 00000010  2f 70 6c 61 74 66 6f 72  6d 2f 73 6f 63 2f 32 31  |/platform/soc/21|
>>>> 00000020  38 30 30 30 30 2e 69 32  63 2f 69 32 63 2d 30 2f  |80000.i2c/i2c-0/|
>>>> 00000030  30 2d 30 30 35 34 2f 30  2d 30 30 35 34 30 00 00  |0-0054/0-00540..|
>>>> 00000040  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
>>>> *
>>>> 00000070  00 00 00 00 00 00 00 00  00 00 00 00 00 00 ff ff  |................|
>>>> 00000080  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
>>>> *
>>>> 000000f0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 ff 5b  |...............[|
>>>> 00000100
>>>>
>>>> (patch with my debug prints appended below)
>>>>
>>>> Despite the DMA completing successfully, no data was copied into the
>>>> buffer, leaving the original (now junk) contents. I probed the I2C bus
>>>> with an oscilloscope, and I verified that the transfer did indeed occur.
>>>> The timing between submission and completion seems reasonable for the
>>>> bus speed (50 kHz for whatever reason).
>>>>
>>>> I had a look over the I2C driver, and nothing looked obviously
>>>> incorrect. If anyone has ideas on what to try, I'm more than willing.
>>> 
>>> Is the DMA controller cache-coherent? I see the mainline LS1046A DT doesn't have a "dma-coherent" property for it, but the behaviour is entirely consistent with that being wrong - dma_map_single() cleans the cache, coherent DMA write hits the still-present cache lines, dma_unmap_single() invalidates the cache, and boom, the data is gone and you read back the previous content of the buffer that was cleaned out to DRAM beforehand.
>> 
>> I've tried both with and without [1] applied. I also tried removing the
>> call to dma_unmap_single, but to no effect.
> 
> Actually, I wasn't updating my device tree like I thought...
> 
> Turns out I2C works only *without* this patch.
> 
> So maybe the eDMA is not coherent?

It seems like this might be the case. From the reference manual:

> All transactions from eDMA are tagged as snoop configuration if the
> SCFG_SNPCNFGCR[eDMASNP] bit is set. Refer Snoop Configuration Register
> (SCFG_SNPCNFGCR) for details.

But there is no such bit in this register on the LS1046A. On the
LS1043A, this bit does exist, but on the LS1046A it is reserved. I read
the register, and found the bit was 0. Perhaps eDMA was intended to be
coherent, but there was a hardware bug?

If this is the case, then I think dma-coherent should be left out of the
top-level /soc node. Instead, the qdma, sata, usb, and emmc nodes should
have dma-coherent added.

Li/Laurentiu, what are your thoughts?

--Sean