linux-kernel - Re: [PATCH v1 tty] 8250: microchip: pci1xxxx: Refactor TX Burst code to use pre-existing APIs

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Date: Mon, 4 Mar 2024 04:37:11 +0000
From: <Rengarajan.S@...rochip.com>
To: <jirislaby@...nel.org>, <linux-serial@...r.kernel.org>,
	<gregkh@...uxfoundation.org>, <Kumaravel.Thiagarajan@...rochip.com>,
	<UNGLinuxDriver@...rochip.com>, <Tharunkumar.Pasumarthi@...rochip.com>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 tty] 8250: microchip: pci1xxxx: Refactor TX Burst code
 to use pre-existing APIs

Hi Jiri,

On Fri, 2024-02-23 at 10:26 +0100, Jiri Slaby wrote:
> EXTERNAL EMAIL: Do not click links or open attachments unless you
> know the content is safe
> 
> On 23. 02. 24, 10:21, Rengarajan.S@...rochip.com wrote:
> > On Fri, 2024-02-23 at 07:08 +0100, Jiri Slaby wrote:
> > > EXTERNAL EMAIL: Do not click links or open attachments unless you
> > > know the content is safe
> > > 
> > > On 22. 02. 24, 14:49, Rengarajan S wrote:
> > > > Updated the TX Burst implementation by changing the circular
> > > > buffer
> > > > processing with the pre-existing APIs in kernel. Also updated
> > > > conditional
> > > > statements and alignment issues for better readability.
> > > 
> > > Hi,
> > > 
> > > so why are you keeping the nested double loop?
> > > 
> > 
> > Hi, in order to differentiate Burst mode handling with byte mode
> > had
> > seperate loops for both. Since, having single while loop also does
> > not
> > align with rx implementation (where we have seperate handling for
> > burst
> > and byte) have retained the double loop.
> 
> So obviously, align RX to a single loop if possible. The current TX
> code
> is very hard to follow and sort of unmaintainable (and buggy). And
> IMO
> it's unnecessary as I proposed [1]. And even if RX cannot be one
> loop,
> you still can make TX easy to read as the two need not be the same.
> 
> [1]
> https://lore.kernel.org/all/b8325c3f-bf5b-4c55-8dce-ef395edce251@kernel.org/


while (data_empty_count) {
   cnt = CIRC_CNT_TO_END();
   if (!cnt)
     break;
   if (cnt < UART_BURST_SIZE || (tail & 3)) { // is_unaligned()
     writeb();
     cnt = 1;
   } else {
     writel()
     cnt = UART_BURST_SIZE;
   }
   uart_xmit_advance(cnt);
   data_empty_count -= cnt;
}

With the above implementation we are observing performance drop of 2
Mbps at baud rate of 4 Mbps. The reason for this is the fact that for
each iteration we are checking if the the data need to be processed via
DWORDs or Bytes. The condition check for each iteration is causing the
drop in performance.

With the previous implementation(with nested loops) the performance is
found to be around 4 Mbps at baud rate of 4 Mbps. In that
implementation we handle sending DWORDs continuosly until the transfer
size < 4. Can you let us know any other alternatives for the above
performance drop.


> 
> thanks,
> --
> js
> suse labs
>