Date:   Thu, 30 Jan 2020 15:19:08 +0200
From:   Peter Ujfalusi <peter.ujfalusi@...com>
To:     Vinod Koul <vkoul@...nel.org>
CC:     <dmaengine@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <dan.j.williams@...el.com>, <grygorii.strashko@...com>,
        <vigneshr@...com>
Subject: Re: [PATCH for-next 0/4] dmaengine: ti: k3-udma: Updates for next

Hi Vinod,

On 28/01/2020 14.37, Peter Ujfalusi wrote:
> Hi Vinod,
> 
> On 28/01/2020 13.50, Vinod Koul wrote:
>> On 28-01-20, 12:15, Peter Ujfalusi wrote:
>>> Vinod,
>>>
>>> On 27/01/2020 15.21, Peter Ujfalusi wrote:
>>>> Hi Vinod,
>>>>
>>>> Based on customer reports we have identified two issues with the UDMA driver:
>>>>
>>>> TX completion (1st patch):
>>>> The scheduled work based workaround for checking for completion worked well for
>>>> UART, but it had significant impact on SPI performance.
>>>> The underlying issue is coming from the fact that we have split data movement
>>>> architecture.
>>>> In order to know that the transfer is really done we need to check the remote
>>>> end's (PDMA) byte counter.
>>>>
>>>> RX channel teardown with stale data in PDMA (2nd patch):
>>>> If we try to stop the RX DMA channel (teardown) then PDMA is trying to flush the
>>>> data is might received from a peripheral, but if UDMA does not have a packet to
>>>> use for this draining than it is going to push back on the PDMA and the flush
>>>> will never completes.
>>>> The workaround is to use a dummy descriptor for flush purposes when the channel
>>>> is terminated and we did not have active transfer (no descriptor for UDMA).
>>>> This allows UDMA to drain the data and the teardown can complete.
>>>>
>>>> The last two patch is to use common code to set up the TR parameters for
>>>> slave_sg, cyclic and memcpy. The setup code is the same as we used for memcpy
>>>> with the change we can handle 4.2GB sg elements and periods in case of cyclic.
>>>> It is also nice that we have single function to do the configuration.
>>>
>>> I have marked these patches as for-next as 5.5 was not released yet.
>>> Would it be possible to have these as fixes for 5.6?
>>
>> Sure but are they really fixes, why cant they go for next release :)
>>
>> They seem to improve things for sure, but do we want to call them as
>> fixes..?
> 
> I would say that the first two patch is a fix:
> TX completion check is fixing the performance hit by the early TX
> completion workaround which used jiffies+work.
> 
> The second patch is fixing a case when we have stale data during RX and
> no active transfer. For example when UART reads 1000 bytes, but the
> other end is 'streaming' the data and after the 1000 bytes the UART+PDMA
> receives data.
> Recovering from this state is not easy and it might not even succeed in
> HW level.
> 
> The last two is I agree, it is not fixing much, it does corrects the
> slave_sg TR setup (and improves the cyclic as well).
> With that I could send the ASoC platform wrapper for UDMA with
> period_bytes_max = 4.2GB ;)
> I have SZ_512K in there atm, with the old calculation SZ_64K is the
> maximum, not a big issue.

Actually this also fixes a real bug in the driver for the slave_sg_tr case:
if the sg_dma_len(sgent) is not multiple of (burst * dev_width) then we
end up with missing bits as the counters are not set up correctly.
The client driver which we tested the slave_sg_tr was always giving
sg_len == 1 and the buffer was aligned, but when I tuned the client to
pass a list, things got broken.

> 
> I think the first two patch is a fix candidate as they fix regression
> (albeit regression between the series's) and a real world channel lockup
> discovered too late for the initial driver.
> 
> - Péter
> 
> Texas Instruments Finland Oy, Porkkalankatu 22, 00180 Helsinki.
> Y-tunnus/Business ID: 0615521-4. Kotipaikka/Domicile: Helsinki
> 

- Péter

Texas Instruments Finland Oy, Porkkalankatu 22, 00180 Helsinki.
Y-tunnus/Business ID: 0615521-4. Kotipaikka/Domicile: Helsinki

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