mtd: rawnand: fsmc: Keep bank enable bit set

Hammering the "bank enable" (PBKEN) bit on and off between
every command crashes the Nomadik NHK15 with this message:

Scanning device for bad blocks
Unhandled fault: external abort on non-linefetch (0x008) at 0xcc95e000
pgd = (ptrval)
[cc95e000] *pgd=0b808811, *pte=40000653, *ppte=40000552
Internal error: : 8 [#1] PREEMPT ARM
Modules linked in:
CPU: 0 PID: 1 Comm: swapper Not tainted 4.20.0-rc2+ #72
Hardware name: Nomadik STn8815
PC is at fsmc_exec_op+0x194/0x204
(...)

After a discussion we (me and Boris Brezillon) start to suspect
that this bit does not immediately control the chip select line
at all, it rather enables access to the bank and the hardware
will drive the CS autonomously. If there is a NAND chip connected,
we should keep this enabled.

As fsmc_nand_setup() sets this bit, we can simply remove the
offending code.

Fixes: 550b9fc4 ("mtd: rawnand: fsmc: Stop implementing ->select_chip()")
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Boris Brezillon <bbrezillon@kernel.org>

drivers/mtd/nand/raw/fsmc_nand.c

@@ -593,23 +593,6 @@ static void fsmc_write_buf_dma(struct fsmc_nand_data *host, const u8 *buf,
 	dma_xfer(host, (void *)buf, len, DMA_TO_DEVICE);
 }
 
-/* fsmc_select_chip - assert or deassert nCE */
-static void fsmc_ce_ctrl(struct fsmc_nand_data *host, bool assert)
-{
-	u32 pc = readl(host->regs_va + FSMC_PC);
-
-	if (!assert)
-		writel_relaxed(pc & ~FSMC_ENABLE, host->regs_va + FSMC_PC);
-	else
-		writel_relaxed(pc | FSMC_ENABLE, host->regs_va + FSMC_PC);
-
-	/*
-	 * nCE line changes must be applied before returning from this
-	 * function.
-	 */
-	mb();
-}
-
 /*
  * fsmc_exec_op - hook called by the core to execute NAND operations
  *
@@ -627,8 +610,6 @@ static int fsmc_exec_op(struct nand_chip *chip, const struct nand_operation *op,
 
 	pr_debug("Executing operation [%d instructions]:\n", op->ninstrs);
 
-	fsmc_ce_ctrl(host, true);
-
 	for (op_id = 0; op_id < op->ninstrs; op_id++) {
 		instr = &op->instrs[op_id];
 
@@ -686,8 +667,6 @@ static int fsmc_exec_op(struct nand_chip *chip, const struct nand_operation *op,
 		}
 	}
 
-	fsmc_ce_ctrl(host, false);
-
 	return ret;
 }