linux-kernel - RE: [PATCH V5 1/4] spi: tegra114: add support for gpio based CS

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Date:   Tue, 14 May 2019 17:18:48 +0000
From:   Sowjanya Komatineni <skomatineni@...dia.com>
To:     Jonathan Hunter <jonathanh@...dia.com>,
        "thierry.reding@...il.com" <thierry.reding@...il.com>,
        Laxman Dewangan <ldewangan@...dia.com>,
        "broonie@...nel.org" <broonie@...nel.org>,
        Krishna Yarlagadda <kyarlagadda@...dia.com>
CC:     "linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-spi@...r.kernel.org" <linux-spi@...r.kernel.org>
Subject: RE: [PATCH V5 1/4] spi: tegra114: add support for gpio based CS

> Subject: Re: [PATCH V5 1/4] spi: tegra114: add support for gpio based CS

> On 14/05/2019 06:03, Sowjanya Komatineni wrote:
> > This patch adds support for GPIO based CS control through SPI core 
> > function spi_set_cs.
> > 
> > Signed-off-by: Sowjanya Komatineni <skomatineni@...dia.com>
> Can you elaborate on the use-case where this is needed? I am curious what platforms are using this and why they would not use the dedicated CS signals.
>
> Cheers
> Jon

Tegra SPI doesn’t support inter byte delay directly to meet some SPI slave requirements.
So we use GPIO control CS in parallel with a dummy HW CS and use inactive cycles delay of SPI controller to mimic inter byte delay.

Currently we don’t have specific SPI slave on upstream supported platforms but considering raspberry PI header where SPI I/F is exposed to pins it allows user to connect any SPI slave and this helps for some slaves that need specific inter byte delay.

Thanks
sowjanya