[Tech] DMA mode: ci fa o ci è?!?

[Alessandro Zarrilli]

Ecco un'estratto dell'output di "dmesg":

    ide0: BM-DMA at 0xd800-0xd807, BIOS settings: hda:DMA, hdb:pio
ide0: VIA Bus-Master (U)DMA Timing Config Success
    ide1: BM-DMA at 0xd808-0xd80f, BIOS settings: hdc:DMA, hdd:pio
ide1: VIA Bus-Master (U)DMA Timing Config Success
hda: QUANTUM FIREBALLP LM20.5, ATA DISK drive
hdc: WT3244EI, ATAPI CDROM drive
ide0 at 0x1f0-0x1f7,0x3f6 on irq 14
ide1 at 0x170-0x177,0x376 on irq 15
hda: QUANTUM FIREBALLP LM20.5, 19595MB w/1900kB Cache, CHS=2498/255/63,
UDMA(66)

Guardando l'ultima riga sembrerebbe che il mio hard disk stia
funzionando in modalità UDMA66, come è giusto che sia. Controllando
ulteriormente con "hdparm /dev/hda":

/dev/hda:
 multcount    = 16 (on)
 I/O support  =  1 (32-bit)
 unmaskirq    =  0 (off)
 using_dma    =  1 (on)
 keepsettings =  0 (off)
 nowerr       =  0 (off)
 readonly     =  0 (off)
 readahead    =  8 (on)
 geometry     = 2498/255/63, sectors = 40132503, start = 0

Perfetto, la modalità DMA è abilitata. Ed allora cosa vuol dire
questo?!? ("cat /proc/ide/via"):

Command register = 0x87
Master Read  Cycle IRDY 0 Wait State
Master Write Cycle IRDY 0 Wait State
FIFO Output Data 1/2 Clock Advance: off
Bus Master IDE Status Register Read Retry: on
Latency timer = 32 (max. = 0)
Interrupt Steering Swap: off
------------------Primary IDE------------Secondary IDE-----
both channels togth:   yes                     yes
Prefetch Buffer :      on                      on
Post Write Buffer:     on                      on
FIFO Conf/Chan. :      08                      08
Threshold Prim. :      1/2                     1/2
Read DMA FIFO flush:   on                      on
End Sect. FIFO flush:  on                      on
Max DRDY Pulse Width:  No limitation
Bytes Per Sector:      512                     512
--------------drive0------drive1-------drive0------drive1----
DMA enabled:    yes         no           yes         no
Act Pls Width:  03          11           03          11
Recovery Time:  01          09           01          09
Add. Setup T.:  4T          4T           4T          4T
------------------UDMA-Timing-Control------------------------
Enable Meth.:    1           0            0           0
Enable:         yes         no           no          no
Transfer Mode: PIO         DMA          DMA         DMA
Cycle Time:     2T          5T           5T          5T

Cosa significa quel PIO nella penultima riga? Non dovrebbe esserci un
bel DMA?!? Illuminatemi, vi prego...