linux-kernel - Re: [PATCHv5] drivers/amba: add reset control to amba bus probe

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Message-ID: <46bcf493-9dd6-bf5b-089a-be158739a13f@kernel.org>
Date:   Wed, 28 Aug 2019 08:34:20 -0500
From:   Dinh Nguyen <dinguyen@...nel.org>
To:     Valdis Klētnieks <valdis.kletnieks@...edu>
Cc:     linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
        robh@...nel.org, linux@...linux.org.uk, frowand.list@...il.com,
        keescook@...omium.org, anton@...msg.org, ccross@...roid.com,
        tony.luck@...el.com, daniel.thompson@...aro.org,
        linus.walleij@...aro.org, manivannan.sadhasivam@...aro.org,
        linux-arm-kernel@...ts.infradead.org, p.zabel@...gutronix.de
Subject: Re: [PATCHv5] drivers/amba: add reset control to amba bus probe

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On 8/27/19 2:25 PM, Valdis Kl?tnieks wrote:
> On Mon, 26 Aug 2019 10:42:52 -0500, Dinh Nguyen said:
>> The primecell controller on some SoCs, i.e. SoCFPGA, is held in
>> reset by default. Until recently, the DMA controller was brought
>> out of reset by the bootloader(i.e. U-Boot). But a recent change
>> in U-Boot, the peripherals that are not used are held in reset
>> and are left to Linux to bring them out of reset.
>> 
>> Add a mechanism for getting the reset property and de-assert the
>> primecell module from reset if found. This is a not a hard fail
>> if the reset properti is not present in the device tree node, so
>> the driver will continue to probe.
> 
> Does this DTRT for both old and new U-Boots? My naive reading of
> this patch

What is a DTRT?

> says on an old U-Boot, we end up attempting to bring it out of
> reset even though they had already been brought out.
> 

If the peripheral is already out of reset, de-asserting the reset has
no affect.

Dinh

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