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diff --git a/Documentation/mtd/nand/pxa3xx-nand.txt b/Documentation/mtd/nand/pxa3xx-nand.txt new file mode 100644 index 000000000..1074cbc67 --- /dev/null +++ b/Documentation/mtd/nand/pxa3xx-nand.txt @@ -0,0 +1,113 @@ + +About this document +=================== + +Some notes about Marvell's NAND controller available in PXA and Armada 370/XP +SoC (aka NFCv1 and NFCv2), with an emphasis on the latter. + +NFCv2 controller background +=========================== + +The controller has a 2176 bytes FIFO buffer. Therefore, in order to support +larger pages, I/O operations on 4 KiB and 8 KiB pages is done with a set of +chunked transfers. + +For instance, if we choose a 2048 data chunk and set "BCH" ECC (see below) +we'll have this layout in the pages: + + ------------------------------------------------------------------------------ + | 2048B data | 32B spare | 30B ECC || 2048B data | 32B spare | 30B ECC | ... | + ------------------------------------------------------------------------------ + +The driver reads the data and spare portions independently and builds an internal +buffer with this layout (in the 4 KiB page case): + + ------------------------------------------ + | 4096B data | 64B spare | + ------------------------------------------ + +Also, for the READOOB command the driver disables the ECC and reads a 'spare + ECC' +OOB, one per chunk read. + + ------------------------------------------------------------------- + | 4096B data | 32B spare | 30B ECC | 32B spare | 30B ECC | + ------------------------------------------------------------------- + +So, in order to achieve reading (for instance), we issue several READ0 commands +(with some additional controller-specific magic) and read two chunks of 2080B +(2048 data + 32 spare) each. +The driver accommodates this data to expose the NAND core a contiguous buffer +(4096 data + spare) or (4096 + spare + ECC + spare + ECC). + +ECC +=== + +The controller has built-in hardware ECC capabilities. In addition it is +configurable between two modes: 1) Hamming, 2) BCH. + +Note that the actual BCH mode: BCH-4 or BCH-8 will depend on the way +the controller is configured to transfer the data. + +In the BCH mode the ECC code will be calculated for each transferred chunk +and expected to be located (when reading/programming) right after the spare +bytes as the figure above shows. + +So, repeating the above scheme, a 2048B data chunk will be followed by 32B +spare, and then the ECC controller will read/write the ECC code (30B in +this case): + + ------------------------------------ + | 2048B data | 32B spare | 30B ECC | + ------------------------------------ + +If the ECC mode is 'BCH' then the ECC is *always* 30 bytes long. +If the ECC mode is 'Hamming' the ECC is 6 bytes long, for each 512B block. +So in Hamming mode, a 2048B page will have a 24B ECC. + +Despite all of the above, the controller requires the driver to only read or +write in multiples of 8-bytes, because the data buffer is 64-bits. + +OOB +=== + +Because of the above scheme, and because the "spare" OOB is really located in +the middle of a page, spare OOB cannot be read or write independently of the +data area. In other words, in order to read the OOB (aka READOOB), the entire +page (aka READ0) has to be read. + +In the same sense, in order to write to the spare OOB the driver has to write +an *entire* page. + +Factory bad blocks handling +=========================== + +Given the ECC BCH requires to layout the device's pages in a split +data/OOB/data/OOB way, the controller has a view of the flash page that's +different from the specified (aka the manufacturer's) view. In other words, + +Factory view: + + ----------------------------------------------- + | Data |x OOB | + ----------------------------------------------- + +Driver's view: + + ----------------------------------------------- + | Data | OOB | Data x | OOB | + ----------------------------------------------- + +It can be seen from the above, that the factory bad block marker must be +searched within the 'data' region, and not in the usual OOB region. + +In addition, this means under regular usage the driver will write such +position (since it belongs to the data region) and every used block is +likely to be marked as bad. + +For this reason, marking the block as bad in the OOB is explicitly +disabled by using the NAND_BBT_NO_OOB_BBM option in the driver. The rationale +for this is that there's no point in marking a block as bad, because good +blocks are also 'marked as bad' (in the OOB BBM sense) under normal usage. + +Instead, the driver relies on the bad block table alone, and should only perform +the bad block scan on the very first time (when the device hasn't been used). |