linux-kernel - Re: [PATCH 5/6] spi: imx: support dynamic burst length for ECSPI DMA mode

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Message-ID:
 <VI2PR04MB111471DB8E08CC180DA90589FE8DEA@VI2PR04MB11147.eurprd04.prod.outlook.com>
Date: Wed, 26 Nov 2025 11:17:41 +0000
From: Carlos Song <carlos.song@....com>
To: Marc Kleine-Budde <mkl@...gutronix.de>
CC: "broonie@...nel.org" <broonie@...nel.org>, Frank Li <frank.li@....com>,
	"hawnguo@...nel.org" <hawnguo@...nel.org>, "s.hauer@...gutronix.de"
	<s.hauer@...gutronix.de>, "kernel@...gutronix.de" <kernel@...gutronix.de>,
	"festevam@...il.com" <festevam@...il.com>, "imx@...ts.linux.dev"
	<imx@...ts.linux.dev>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "linux-arm-kernel@...ts.infradead.org"
	<linux-arm-kernel@...ts.infradead.org>, "linux-spi@...r.kernel.org"
	<linux-spi@...r.kernel.org>
Subject: Re: [PATCH 5/6] spi: imx: support dynamic burst length for ECSPI DMA
 mode



> -----Original Message-----
> From: Marc Kleine-Budde <mkl@...gutronix.de>
> Sent: Wednesday, November 26, 2025 4:31 PM
> To: Carlos Song <carlos.song@....com>
> Cc: broonie@...nel.org; Frank Li <frank.li@....com>; hawnguo@...nel.org;
> s.hauer@...gutronix.de; kernel@...gutronix.de; festevam@...il.com;
> imx@...ts.linux.dev; linux-kernel@...r.kernel.org;
> linux-arm-kernel@...ts.infradead.org; linux-spi@...r.kernel.org
> Subject: [EXT] Re: [PATCH 5/6] spi: imx: support dynamic burst length for ECSPI
> DMA mode
> 
> On 26.11.2025 07:42:36, Carlos Song wrote:
> 
> [...]
> > For example len is 512 *3 + 511. So first transfer using BURST_LENGTH
> > = 512 bytes(auto update 3 times), DMA transfer len = 512 *3, second
> > package BURST_LENGTH = 511 bytes, DMA transfer len = 511.(If here we
> > use 512 bytes as BURST_LENGTH, SPI will shift out/in extra bits, it
> > previous isn't acceptable!)
> 
> What happens if you keep the Burst Length at 512 and only transfer 511 bytes
> with the DMA engine?
> 

Sorry for missing one question:
BURST_LENGTH = 511, ECSPI will shift out 511 bytes in bus.
BURST_LENGTH = 512, ECSPI will shift out the last one word all bit in 32 bits FIFO. 
So ECSPI will shift out 512 bytes include 8-bits zero bytes in bus.

Carlos

> Marc
> 
> --
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