linux-kernel - Re: [PATCH v2] mtd: rawnand: denali: add more delays before latching incoming data

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Message-ID: <6093dfab-1e9e-824a-b639-33d340b377f9@denx.de>
Date:   Wed, 22 Apr 2020 17:29:53 +0200
From:   Marek Vasut <marex@...x.de>
To:     Masahiro Yamada <yamada.masahiro@...ionext.com>,
        linux-mtd@...ts.infradead.org
Cc:     Miquel Raynal <miquel.raynal@...tlin.com>,
        Richard Weinberger <richard@....at>,
        Vignesh Raghavendra <vigneshr@...com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mtd: rawnand: denali: add more delays before latching
 incoming data

On 3/17/20 8:18 AM, Masahiro Yamada wrote:
> The Denali IP have several registers to specify how many clock cycles
> should be waited between falling/rising signals. You can improve the
> NAND access performance by programming these registers with optimized
> values.
> 
> Because struct nand_sdr_timings represents the device requirement
> in pico seconds, denali_setup_data_interface() computes the register
> values by dividing the device timings with the clock period.
> 
> Marek Vasut reported this driver in the latest kernel does not work
> on his SOCFPGA board. (The on-board NAND chip is mode 5)
> 
> The suspicious parameter is acc_clks, so this commit relaxes it.
> 
> The Denali NAND Flash Memory Controller User's Guide describes this
> register as follows:
> 
>   acc_clks
>     signifies the number of bus interface clk_x clock cycles,
>     controller should wait from read enable going low to sending
>     out a strobe of clk_x for capturing of incoming data.
> 
> Currently, acc_clks is calculated only based on tREA, the delay on the
> chip side. This does not include additional delays that come from the
> data path on the PCB and in the SoC, load capacity of the pins, etc.
> 
> This relatively becomes a big factor on faster timing modes like mode 5.
> 
> Before supporting the ->setup_data_interface() hook (e.g. Linux 4.12),
> the Denali driver hacks acc_clks in a couple of ways [1] [2] to support
> the timing mode 5.
> 
> We would not go back to the hard-coded acc_clks, but we need to include
> this factor into the delay somehow. Let's say the amount of the additional
> delay is 10000 pico sec.
> 
> In the new calculation, acc_clks is determined by timings->tREA_max +
> data_setup_on_host.
> 
> Also, prolong the RE# low period to make sure the data hold is met.
> 
> Finally, re-center the data latch timing for extra safety.
> 
> [1] https://github.com/torvalds/linux/blob/v4.12/drivers/mtd/nand/denali.c#L276
> [2] https://github.com/torvalds/linux/blob/v4.12/drivers/mtd/nand/denali.c#L282
> 
> Reported-by: Marek Vasut <marex@...x.de>
> Signed-off-by: Masahiro Yamada <yamada.masahiro@...ionext.com>

I tested it on the AV SoCFPGA, this seems to work, so feel free to apply.