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Date:   Tue, 12 Jun 2018 11:17:09 +0200
From:   Stefan Agner <stefan@agner.ch>
To:     Boris Brezillon <boris.brezillon@bootlin.com>, axboe@kernel.dk
Cc:     Dmitry Osipenko <digetx@gmail.com>, dwmw2@infradead.org,
        computersforpeace@gmail.com, marek.vasut@gmail.com,
        robh+dt@kernel.org, mark.rutland@arm.com, thierry.reding@gmail.com,
        dev@lynxeye.de, miquel.raynal@bootlin.com, richard@nod.at,
        marcel@ziswiler.com, krzk@kernel.org, benjamin.lindqvist@endian.se,
        jonathanh@nvidia.com, pdeschrijver@nvidia.com, pgaikwad@nvidia.com,
        mirza.krak@gmail.com, gaireg@gaireg.de,
        linux-mtd@lists.infradead.org, linux-tegra@vger.kernel.org,
        devicetree@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4 4/6] mtd: rawnand: add NVIDIA Tegra NAND Flash
 controller driver
In-Reply-To: <20180612102734.0ea3bfa5@bbrezillon>
References: <20180611205224.23340-1-stefan@agner.ch>
 <20180611205224.23340-5-stefan@agner.ch> <4258393.O1xtUOGbca@dimapc>
 <cb6f2ef0a5a41e001a2019caa1a18c23@agner.ch>
 <20180612102734.0ea3bfa5@bbrezillon>
Message-ID: <53e36cff80e38e489959aa18471d0782@agner.ch>
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[also added Jens Axboe]

On 12.06.2018 10:27, Boris Brezillon wrote:
> On Tue, 12 Jun 2018 10:06:42 +0200
> Stefan Agner <stefan@agner.ch> wrote:
> 
>> On 12.06.2018 02:03, Dmitry Osipenko wrote:
>> > On Monday, 11 June 2018 23:52:22 MSK Stefan Agner wrote:
>> >> Add support for the NAND flash controller found on NVIDIA
>> >> Tegra 2 SoCs. This implementation does not make use of the
>> >> command queue feature. Regular operations/data transfers are
>> >> done in PIO mode. Page read/writes with hardware ECC make
>> >> use of the DMA for data transfer.
>> >>
>> >> Signed-off-by: Lucas Stach <dev@lynxeye.de>
>> >> Signed-off-by: Stefan Agner <stefan@agner.ch>
>> >> ---
>> >>  MAINTAINERS                       |    7 +
>> >>  drivers/mtd/nand/raw/Kconfig      |    6 +
>> >>  drivers/mtd/nand/raw/Makefile     |    1 +
>> >>  drivers/mtd/nand/raw/tegra_nand.c | 1248 +++++++++++++++++++++++++++++
>> >>  4 files changed, 1262 insertions(+)
>> >>  create mode 100644 drivers/mtd/nand/raw/tegra_nand.c
>> >>
>> [snip]
>> >> +static int tegra_nand_cmd(struct nand_chip *chip,
>> >> +			 const struct nand_subop *subop)
>> >> +{
>> >> +	const struct nand_op_instr *instr;
>> >> +	const struct nand_op_instr *instr_data_in = NULL;
>> >> +	struct tegra_nand_controller *ctrl = to_tegra_ctrl(chip->controller);
>> >> +	unsigned int op_id, size = 0, offset = 0;
>> >> +	bool first_cmd = true;
>> >> +	u32 reg, cmd = 0;
>> >> +	int ret;
>> >> +
>> >> +	for (op_id = 0; op_id < subop->ninstrs; op_id++) {
>> >> +		unsigned int naddrs, i;
>> >> +		const u8 *addrs;
>> >> +		u32 addr1 = 0, addr2 = 0;
>> >> +
>> >> +		instr = &subop->instrs[op_id];
>> >> +
>> >> +		switch (instr->type) {
>> >> +		case NAND_OP_CMD_INSTR:
>> >> +			if (first_cmd) {
>> >> +				cmd |= COMMAND_CLE;
>> >> +				writel_relaxed(instr->ctx.cmd.opcode,
>> >> +					       ctrl->regs + CMD_REG1);
>> >> +			} else {
>> >> +				cmd |= COMMAND_SEC_CMD;
>> >> +				writel_relaxed(instr->ctx.cmd.opcode,
>> >> +					       ctrl->regs + CMD_REG2);
>> >> +			}
>> >> +			first_cmd = false;
>> >> +			break;
>> >> +		case NAND_OP_ADDR_INSTR:
>> >> +			offset = nand_subop_get_addr_start_off(subop, op_id);
>> >> +			naddrs = nand_subop_get_num_addr_cyc(subop, op_id);
>> >> +			addrs = &instr->ctx.addr.addrs[offset];
>> >> +
>> >> +			cmd |= COMMAND_ALE | COMMAND_ALE_SIZE(naddrs);
>> >> +			for (i = 0; i < min_t(unsigned int, 4, naddrs); i++)
>> >> +				addr1 |= *addrs++ << (BITS_PER_BYTE * i);
>> >> +			naddrs -= i;
>> >> +			for (i = 0; i < min_t(unsigned int, 4, naddrs); i++)
>> >> +				addr2 |= *addrs++ << (BITS_PER_BYTE * i);
>> >> +			writel_relaxed(addr1, ctrl->regs + ADDR_REG1);
>> >> +			writel_relaxed(addr2, ctrl->regs + ADDR_REG2);
>> >> +			break;
>> >> +
>> >> +		case NAND_OP_DATA_IN_INSTR:
>> >> +			size = nand_subop_get_data_len(subop, op_id);
>> >> +			offset = nand_subop_get_data_start_off(subop, op_id);
>> >> +
>> >> +			cmd |= COMMAND_TRANS_SIZE(size) | COMMAND_PIO |
>> >> +				COMMAND_RX | COMMAND_A_VALID;
>> >> +
>> >> +			instr_data_in = instr;
>> >> +			break;
>> >> +
>> >> +		case NAND_OP_DATA_OUT_INSTR:
>> >> +			size = nand_subop_get_data_len(subop, op_id);
>> >> +			offset = nand_subop_get_data_start_off(subop, op_id);
>> >> +
>> >> +			cmd |= COMMAND_TRANS_SIZE(size) | COMMAND_PIO |
>> >> +				COMMAND_TX | COMMAND_A_VALID;
>> >> +
>> >> +			memcpy(&reg, instr->ctx.data.buf.out + offset, size);
>> >> +			writel_relaxed(reg, ctrl->regs + RESP);
>> >> +
>> >> +			break;
>> >> +		case NAND_OP_WAITRDY_INSTR:
>> >> +			cmd |= COMMAND_RBSY_CHK;
>> >> +			break;
>> >> +
>> >> +		}
>> >> +	}
>> >> +
>> >> +	cmd |= COMMAND_GO | COMMAND_CE(ctrl->cur_cs);
>> >> +	writel_relaxed(cmd, ctrl->regs + COMMAND);
>> >> +	ret = wait_for_completion_io_timeout(&ctrl->command_complete,
>> >> +					     msecs_to_jiffies(500));
>> >
>> > It's not obvious to me whether _io_ variant is appropriate to use here, would
>> > be nice if somebody could clarify that. Maybe block/ already does the IO
>> > accounting itself and hence the IO time would be counted twice in that case.
>>
>> Good that you bring this up.
>>
>> I don't think that there is any higher layer which could take care of
>> accounting. Usually, with raw nand there is no block layer involved
>> anyway.
>>
>> In a quick test it seems that only when using wait_for_completion_io I/O
>> is properly accounted in the "wait" section of top.
>>
>> So far only a single driver (omap2) used the _io variant, but I think it
>> is the right thing to do! After all, it is I/O...
>>
>> Boris or any other MTD maintainer, any comment on this?
> 
> Given this definition of io_schedule_timeout() [1] (which is used when
> you call wait_for_completion_io_timeout()), I'd say it's not useful to
> use the _io_ version, simply because MTD devs are not exposed as blk
> devices, and thus don't need the blk_schedule_flush_plug() that is done
> is io_schedule_prepare(). But that also means MTD I/Os are not
> accounted as I/Os :-(.

Documentation of wait_for_completion_io says:
"The caller is accounted as waiting for IO (which traditionally means
blkio only)."

Which sounds as if it using _io is only an accounting thing...

The hint about blkio might suggest that there is more to it.

Is calling blk_schedule_flush_plug a problem for MTD?
https://elixir.bootlin.com/linux/v4.17.1/source/include/linux/blkdev.h#L1355

> 
> Let's go for the non-io version for now, since all drivers except omap2
> seem to use this function.
> 

I still think it would be nice and "the right thing" from a user
perspective...

--
Stefan

> [1]https://elixir.bootlin.com/linux/v4.17.1/source/kernel/sched/core.c#L5164