linux-kernel - Re: [LINUX PATCH v14] mtd: rawnand: pl353: Add basic driver for arm pl353 smc nand interface

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Date:   Mon, 29 Apr 2019 14:18:06 +0200
From:   Helmut Grohne <helmut.grohne@...enta.de>
To:     Naga Sureshkumar Relli <nagasure@...inx.com>
CC:     "bbrezillon@...nel.org" <bbrezillon@...nel.org>,
        "miquel.raynal@...tlin.com" <miquel.raynal@...tlin.com>,
        "richard@....at" <richard@....at>,
        "dwmw2@...radead.org" <dwmw2@...radead.org>,
        "computersforpeace@...il.com" <computersforpeace@...il.com>,
        "marek.vasut@...il.com" <marek.vasut@...il.com>,
        "linux-mtd@...ts.infradead.org" <linux-mtd@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Michal Simek <michals@...inx.com>,
        "nagasureshkumarrelli@...il.com" <nagasureshkumarrelli@...il.com>
Subject: Re: [LINUX PATCH v14] mtd: rawnand: pl353: Add basic driver for arm
 pl353 smc nand interface

On Mon, Apr 29, 2019 at 11:31:14AM +0000, Naga Sureshkumar Relli wrote:
> But just wanted to know, do you see issues with these __force and __iomem castings?

I only see a minor issue: They're (deliberately) lengthy. Using many of
them diverts attention of the reader. Therefore, my proposal attempted
to reduce their frequency. The only issue I see here is readability.

> > 
> > > +	u8 addr_cycles;
> > > +	struct clk *mclk;
> > 
> > All you need here is the memory clock frequency. Wouldn't it be easier to extract that
> > frequency once during probe and store it here? That assumes a constant frequency, but if the
> > frequency isn't constant, you have a race condition.
> That is what we are doing in the probe.
> In the probe, we are getting mclk using of_clk_get() and then we are getting the actual frequency
> Using clk_get_rate().
> And this is constant frequency only(getting from dts)

Not quite. You're getting a clock reference in probe and then repeatedly
access the frequency elswhere. I am suggesting that you get the clock
frequency during probe and never save the clock reference to a struct.

> > > +		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];
> > > +			nfc_op->addrs = instr->ctx.addr.addrs[offset];
> > > +			for (i = 0; i < min_t(unsigned int, 4, naddrs); i++) {
> > > +				nfc_op->addrs |= instr->ctx.addr.addrs[i] <<
> > 
> > I don't quite understand what this code does, but it looks strange to me. I compared it to other
> > drivers. The code here is quite similar to marvell_nand.c. It seems like we are copying a
> > varying number (0 to 6) of addresses from the buffer instr->ctx.addr.addrs. However their
> > indices are special: 0, 1, 2, 3, offset + 4, offset + 5. This is non-consecutive and different from
> > marvell_nand.c in this regard. Could it be that you really meant index offset+i here?
> I didn't get, what you are saying here.
> It is about updating page and column addresses.
> Are you asking me to remove nfc_op->addrs = instr->ctx.addr.addrs[offset]; before for loop?

I compared this code to marvell_nand.c and noticed a subtle difference.
Both snippets read 6 address bytes and consume them in a driver-specific
way. Now which address bytes are consumed differs.

marvell_nand.c consumes instr->ctx.addr.addrs at indices offset,
offset+1, offset+2, offset+3, offset+4, offset+5. pl353_nand.c consumes
instr->ctx.addr.addrs at indices 0, 1, 2, 3, offset, offset+4, offset+5.
(In my previous mail, I didn't notice that it was also consuming the
offset index.)

I would have expected this behaviour to be consistent between different
drivers. If I assume marvell_nand.c to do the right thing and
pl353_nand.c to be wrong (which is not necessarily a correct
assumption), then the code woule likely becom:

	addrs = &instr->ctx.addr.addrs[offset];
	for (i = 0; i < min_t(unsigned int, 4, naddrs); i++) {
		nfc_op->addrs |= addrs[i] << (8 * i);
		              // ^^^^^
	}

Hope this helps.

Helmut