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

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Date:   Fri, 29 May 2020 12:12:14 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Wesley Cheng <wcheng@...eaurora.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 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...