linux-kernel - Re: [PATCH V2 2/2] mtd: rawnand: qcom: set BAM mode only if not set already

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Date:   Thu, 11 Jun 2020 09:13:36 +0200
From:   Miquel Raynal <miquel.raynal@...tlin.com>
To:     Sivaprakash Murugesan <sivaprak@...eaurora.org>
Cc:     richard@....at, vigneshr@...com, peter.ujfalusi@...com,
        linux-mtd@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2 2/2] mtd: rawnand: qcom: set BAM mode only if not set
 already

Hi Sivaprakash,

Sivaprakash Murugesan <sivaprak@...eaurora.org> wrote on Thu, 11 Jun
2020 09:57:59 +0530:

> Hi Miquel,
> 
> Thanks for the review.
> 
> On 6/9/2020 7:33 PM, Miquel Raynal wrote:
> > Hi Sivaprakash,
> >
> > Sivaprakash Murugesan <sivaprak@...eaurora.org> wrote on Tue,  9 Jun
> > 2020 16:40:56 +0530:
> >  
> >> BAM mode is set by writing BAM_MODE_EN bit on NAND_CTRL register.
> >> NAND_CTRL is an operational register and in BAM mode operational
> >> registers are read only.
> >>
> >> So, before writing into NAND_CTRL register check if BAM mode is already
> >> enabled by bootloader, and set BAM mode only if it is not set already.
> >>
> >> Signed-off-by: Sivaprakash Murugesan <sivaprak@...eaurora.org>
> >> ---
> >>   drivers/mtd/nand/raw/qcom_nandc.c | 9 ++++++++-
> >>   1 file changed, 8 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/mtd/nand/raw/qcom_nandc.c b/drivers/mtd/nand/raw/qcom_nandc.c
> >> index e0afa2c..7740059 100644
> >> --- a/drivers/mtd/nand/raw/qcom_nandc.c
> >> +++ b/drivers/mtd/nand/raw/qcom_nandc.c
> >> @@ -2779,7 +2779,14 @@ static int qcom_nandc_setup(struct qcom_nand_controller *nandc)
> >>   	/* enable ADM or BAM DMA */
> >>   	if (nandc->props->is_bam) {
> >>   		nand_ctrl = nandc_read(nandc, NAND_CTRL);
> >> -		nandc_write(nandc, NAND_CTRL, nand_ctrl | BAM_MODE_EN);
> >> +		/* NAND_CTRL is an operational registers, and CPU
> >> +		 * access to operational registers are read only
> >> +		 * in BAM mode. So update the NAND_CTRL register
> >> +		 * only if it is not in BAM mode. In most cases BAM
> >> +		 * mode will be enabled in bootloader
> >> +		 */
> >> +		if (!(nand_ctrl | BAM_MODE_EN))
> >> +			nandc_write(nandc, NAND_CTRL, nand_ctrl | BAM_MODE_EN);
> >>   	} else {
> >>   		nandc_write(nandc, NAND_FLASH_CHIP_SELECT, DM_EN);
> >>   	}  
> > Does this currently produces an issue at runtime?
> >
> > If yes, you should have a Fixes/CC: stable pair of tags.
> >
> > Also, what is BAM mode? Please tell us in the commit log.  
> 
> Currently this is not causing any issue on run time.
> 
> The writes to this register is silently ignored.
> 
> However, this could be an issue in future Hardware designs.
> 
> BAM is the DMA engine on QCOM IPQ platforms, sure will explain this
> 
> mode in next patchset.

I don't like so much the idea of DMA being enabled by the Bootloader or
not, this is something that should need to be fixed.


Thanks,
Miquèl