Re: [PATCH RFC 6/7] spi: spi-mem: Call spi_set_rx_sampling_point() for each op

From: "Michael Walle" <mwalle@kernel.org>
To: "Miquel Raynal" <miquel.raynal@bootlin.com>,
	"Frieder Schrempf" <frieder.schrempf@kontron.de>
Cc: "Frieder Schrempf" <frieder@fris.de>,
	"Mark Brown" <broonie@kernel.org>,
	"Richard Weinberger" <richard@nod.at>,
	"Vignesh Raghavendra" <vigneshr@ti.com>,
	"Han Xu" <han.xu@nxp.com>,
	"Eberhard Stoll" <eberhard.stoll@kontron.de>,
	"Tudor Ambarus" <tudor.ambarus@linaro.org>,
	"Pratyush Yadav" <pratyush@kernel.org>,
	<linux-spi@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
	<linux-mtd@lists.infradead.org>, <imx@lists.linux.dev>,
	"Santhosh Kumar K" <s-k6@ti.com>
Subject: Re: [PATCH RFC 6/7] spi: spi-mem: Call spi_set_rx_sampling_point() for each op
Date: Wed, 01 Apr 2026 13:00:20 +0200	[thread overview]
Message-ID: <DHHRLJLX9AVS.CRZTDV7TP1DO@kernel.org> (raw)
In-Reply-To: <874ilv84j8.fsf@bootlin.com>

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On Wed Apr 1, 2026 at 10:32 AM CEST, Miquel Raynal wrote:
>> Let's say for a worst case scenario a chip has an RX delay of 20ns (the
>> highest I've seen in datasheets so far is 8ns). In that case the maximum
>> clock we could safely use for reading the ID would be 1/(2*20e-9) =
>> 25MHz. Do you think it really makes much of a difference if we read the
>> ID (only a handful of bytes) at 25MHz or full speed (e. g. 104 MHz)? I
>> mean this should be fast enough either way, no? But maybe I'm misjudging
>> this.
>
> I am honestly not a big fan of the global penalty, but I am not totally
> opposed either, especially since you just said you only observed 8ns
> delays at most. This is 62.5MHz, which is already above what most
> designs use, so the penalty would be minimal. What about taking this
> approach and see if that fixes most of our use cases?

What are the actual numbers we are talking about here? I mean, at
least for SPI NOR, we only read the ID *once*. And it takes about 56
bits (command + id length of 6). That is about 2us at 25MHz. I'd
guess the setup and the software handling takes far longer than
that.

-michael

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