linux-kernel - Re: [RFC v3 0/3] Re-introduce TX FIFO resize for larger EP bursting

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Message-ID: <48776606-518a-77b7-fe44-0bb84df5ce58@codeaurora.org>
Date:   Fri, 29 May 2020 23:31:06 -0700
From:   Wesley Cheng <wcheng@...eaurora.org>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     robh+dt@...nel.org, bjorn.andersson@...aro.org, balbi@...nel.org,
        agross@...nel.org, linux-kernel@...r.kernel.org,
        linux-arm-msm@...r.kernel.org, devicetree@...r.kernel.org,
        linux-usb@...r.kernel.org
Subject: Re: [RFC v3 0/3] Re-introduce TX FIFO resize for larger EP bursting



On 5/29/2020 3:12 AM, Greg KH wrote:
> On Wed, May 27, 2020 at 06:46:00PM -0700, Wesley Cheng wrote:
>> Changes in V3:
>>  - Removed "Reviewed-by" tags
>>  - Renamed series back to RFC
>>  - Modified logic to ensure that fifo_size is reset if we pass the minimum
>>    threshold.  Tested with binding multiple FDs requesting 6 FIFOs.
>>
>> Changes in V2:
>>  - Modified TXFIFO resizing logic to ensure that each EP is reserved a
>>    FIFO.
>>  - Removed dev_dbg() prints and fixed typos from patches
>>  - Added some more description on the dt-bindings commit message
>>
>> Currently, there is no functionality to allow for resizing the TXFIFOs, and
>> relying on the HW default setting for the TXFIFO depth.  In most cases, the
>> HW default is probably sufficient, but for USB compositions that contain
>> multiple functions that require EP bursting, the default settings
>> might not be enough.  Also to note, the current SW will assign an EP to a
>> function driver w/o checking to see if the TXFIFO size for that particular
>> EP is large enough. (this is a problem if there are multiple HW defined
>> values for the TXFIFO size)
>>
>> It is mentioned in the SNPS databook that a minimum of TX FIFO depth = 3
>> is required for an EP that supports bursting.  Otherwise, there may be
>> frequent occurences of bursts ending.  For high bandwidth functions,
>> such as data tethering (protocols that support data aggregation), mass
>> storage, and media transfer protocol (over FFS), the bMaxBurst value can be
>> large, and a bigger TXFIFO depth may prove to be beneficial in terms of USB
>> throughput. (which can be associated to system access latency, etc...)  It
>> allows for a more consistent burst of traffic, w/o any interruptions, as
>> data is readily available in the FIFO.
>>
>> With testing done using the mass storage function driver, the results show
>> that with a larger TXFIFO depth, the bandwidth increased significantly.
> 
> Why is this still a "RFC" series?  That implies you don't want this
> applied...
> 

Hi Greg,

As Felipe mentioned, we need to make sure that this TX FIFO resize logic
is carefully thought out, since the behavior could be different based
off the HW configuration as shown in the past.  Eventually, I hope that
this does get applied, but I think the changes needs more detailed
reviews, as there may be potential shortfalls I did not consider due to
my limited knowledge of what happened w/ the previous logic.  That's
pretty much the reason for tagging it as a RFC, since we still need to
hash out if this is the right approach.

Thanks!

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